Watching the breath during meditation is getting boring?

I don't train according to that book but in the anapanasati sutta there are these instructions;

6. 'I will breathe in sensitive to mental fabrication.' He trains himself, 'I will breathe out sensitive to mental fabrication.'

Mental fabrications are defined;

Perceptions & feelings are mental fabrications." Perceptions & feelings are mental; these are things tied up with the mind. That's why perceptions & feelings are mental fabrications [sn41.006]."

Unpleasant feelings & perceptions associated with boredom would fall into this category.

Furthermore instruction;

8. He trains himself, 'I will breathe in sensitive to the mind.' He trains himself, 'I will breathe out sensitive to the mind.'

How is one sensitive to the mind? That is defined here;

"And how does a monk remain focused on the mind in & of itself? There is the case where a monk, when the mind has passion, discerns that the mind has passion. When the mind is without passion, he discerns that the mind is without passion. When the mind has aversion, he discerns that the mind has aversion. When the mind is without aversion, he discerns that the mind is without aversion. When the mind has delusion, he discerns that the mind has delusion. When the mind is without delusion, he discerns that the mind is without delusion."When the mind is restricted, he discerns that the mind is restricted. When the mind is scattered, he discerns that the mind is scattered. When the mind is enlarged, he discerns that the mind is enlarged. When the mind is not enlarged, he discerns that the mind is not enlarged. When the mind is surpassed, he discerns that the mind is surpassed. When the mind is unsurpassed, he discerns that the mind is unsurpassed. When the mind is concentrated, he discerns that the mind is concentrated. When the mind is not concentrated, he discerns that the mind is not concentrated. When the mind is released, he discerns that the mind is released. When the mind is not released, he discerns that the mind is not released. "In this way he remains focused internally on the mind in & of itself, or externally on the mind in & of itself, or both internally & externally on the mind in & of itself. Or he remains focused on the phenomenon of origination with regard to the mind, on the phenomenon of passing away with regard to the mind, or on the phenomenon of origination & passing away with regard to the mind. Or his mindfulness that 'There is a mind' is maintained to the extent of knowledge & remembrance. And he remains independent, unsustained by (not clinging to) anything in the world. This is how a monk remains focused on the mind in & of itself [DN22]

Therefore when perceptions & feelings associated with boredom arise one should take note of the mind as it is with aversion, longing, delusion etc.

Furthermore instruction;

7. He trains himself, 'I will breathe in calming mental fabrication.' He trains himself; 'I will breathe out calming mental fabrication'

How is this calming achieved? That is by this development;

12. He trains himself, 'I will breathe in focusing on inconstancy.' He trains himself, 'I will breathe out focusing on inconstancy.' 13) He trains himself, 'I will breathe in focusing on dispassion [literally, fading].' He trains himself, 'I will breathe out focusing on dispassion.' 14) He trains himself, 'I will breathe in focusing on cessation.' He trains himself, 'I will breathe out focusing on cessation.' 15) He trains himself, 'I will breathe in focusing on relinquishment.' He trains himself, 'I will breathe out focusing on relinquishment.'[mn.118]

To understand what this refers to consider this;

'Whatever is felt comes under stress.' That I have stated simply in connection with the inconstancy of fabrications. That I have stated simply in connection with the nature of fabrications to end... in connection with the nature of fabrications to fall away... to fade away... to cease... in connection with the nature of fabrications to change [sn36.011].

“On whatever occasion, bhikkhus, a bhikkhu trains thus, ‘I shall breathe in contemplating impermanence’;trains thus, ‘I shall breathe out contemplating impermanence’; trains thus, ‘I shall breathe in contemplating fading away’; trains thus, ‘I shall breathe out contemplating fading away’; trains thus, ‘I shall breathe in contemplating cessation’; trains thus, ‘I shall breathe out contemplating cessation’; trains thus, ‘I shall breathe in contemplating relinquishment’; trains thus, ‘I shall breathe out contemplating relinquishment’—on that occasion, bhikkhus, a bhikkhu abides contemplating mental objects in mental objects, ardent, clearly comprehending, mindful, having put away covetousness and grief regarding the world. [mn.118]

Calming is to be understood thus;

There are these six calmings. When one has attained the first jhāna, speech has been calmed. When one has attained the second jhāna, directed thought & evaluation have been calmed. When one has attained the third jhāna, rapture has been calmed. When one has attained the fourth jhāna, in-and-out breathing has been calmed. When one has attained the cessation of perception & feeling, perception & feeling have been calmed. Sn36

Speech here is a tricky term but here is an alternate progression description;

"When one has attained the first jhāna, the perception of sensuality has been stopped. When one has attained the second jhāna, directed thoughts & evaluations [verbal fabrications] have been stopped. When one has attained the third jhāna, rapture has been stopped. When one has attained the fourth jhāna, in-and-out breaths [bodily fabrications] have been stopped.

As i understand it, as one focuses on the inconstancy and cessation of phenomena one comes to become disintetested & disenchanted with feelings. As one becomes disenchanted the feelings & perceptions are calmed, they are stilled, they cease & fade from the mind.

Furthermore it is said in the mn118;

9. He trains himself, 'I will breathe in satisfying the mind.' He trains himself, 'I will breathe out satisfying the mind.'mn.118

As the feelings & perceptions are calmed one experiences the pleasure & equanimity of renunciation;

And what are the six kinds of renunciation joy? The joy that arises when — experiencing the inconstancy of those very forms, their change, fading, & cessation — one sees with right discernment as it actually is that all forms, past or present, are inconstant, stressful, subject to change: That is called renunciation joy. (Similarly with sounds, smells, tastes, tactile sensations, & ideas.) "And what are the six kinds of renunciation equanimity? The equanimity that arises when — experiencing the inconstancy of those very forms, their change, fading, & cessation — one sees with right discernment as it actually is that all forms, past or present, are inconstant, stressful, subject to change: This equanimity goes beyond form, which is why it is called renunciation equanimity. (Similarly with sounds, smells, tastes, tactile sensations, & ideas.)mn.137

The general development is summed up like this;

"And what is the development of concentration that, when developed & pursued, leads to mindfulness & alertness? There is the case where feelings are known to the monk as they arise, known as they persist, known as they subside. Perceptions are known to him as they arise, known as they persist, known as they subside. Thoughts are known to him as they arise, known as they persist, known as they subside. This is the development of concentration that, when developed & pursued, leads to mindfulness & alertness. [an4.41]

That in itself does not guarantee progress;

"Suppose that there is a foolish, inexperienced, unskillful cook who has presented a king or a king's minister with various kinds of curry: mainly sour, mainly bitter, mainly peppery, mainly sweet, alkaline or non-alkaline, salty or non-salty. He does not take note of[1] his master, thinking, 'Today my master likes this curry, or he reaches out for that curry, or he takes a lot of this curry, or he praises that curry. Today my master likes mainly sour curry... Today my master likes mainly bitter curry... mainly peppery curry... mainly sweet curry... alkaline curry... non-alkaline curry... salty curry... Today my master likes non-salty curry, or he reaches out for non-salty curry, or he takes a lot of non-salty curry, or he praises non-salty curry.' As a result, he is not rewarded with clothing or wages or gifts. Why is that? Because the foolish, inexperienced, unskillful cook does not pick up on the theme of his own master.

"In the same way, there are cases where a foolish, inexperienced, unskillful monk remains focused on the body in & of itself — ardent, alert, & mindful — putting aside greed & distress with reference to the world. As he remains thus focused on the body in & of itself, his mind does not become concentrated, his defilements[2] are not abandoned. He does not take note of that fact.[3] He remains focused on feelings in & of themselves... the mind in & of itself... mental qualities in & of themselves — ardent, alert, & mindful — putting aside greed & distress with reference to the world. As he remains thus focused on mental qualities in & of themselves, his mind does not become concentrated, his defilements are not abandoned. He does not take note of that fact. As a result, he is not rewarded with a pleasant abiding here & now, nor with mindfulness & alertness. Why is that? Because the foolish, inexperienced, unskillful monk does not take note of his own mind.[4] [SN47.08]]

Therefore it is really important to take note of the mind, as i described above and especially with reference to mental qualities in regards to the hindrances;

[1] "There is the case where a monk remains focused on mental qualities in & of themselves with reference to the five hindrances. And how does a monk remain focused on mental qualities in & of themselves with reference to the five hindrances? There is the case where, there being sensual desire present within, a monk discerns that 'There is sensual desire present within me.' Or, there being no sensual desire present within, he discerns that 'There is no sensual desire present within me.' He discerns how there is the arising of unarisen sensual desire. And he discerns how there is the abandoning of sensual desire once it has arisen. And he discerns how there is no future arising of sensual desire that has been abandoned. (The same formula is repeated for the remaining hindrances: ill will, sloth & drowsiness, restlessness & anxiety, and uncertainty.)

If one notices the hindrances one should direct the mind to a different perception which is connected with skillful qualities. In practical terms try focusing more on the inconstancy, cessation or an inspiring theme, what i do depends on the hindrance.

As a result, he is rewarded with a pleasant abiding here & now, together with mindfulness & alertness. Why is that? Because the wise, experienced, skillful monk picks up on the theme of his own mind." [SN47.08]

“Ananda, if a monk or nun remains with mind well-established in the four establishings of mindfulness, he/she may be expected to perceive grand, successive distinctions.

“There is the case of a monk who remains focused on the body in & of itself—ardent, alert, & mindful—subduing greed & distress with reference to the world. As he remains thus focused on the body in & of itself, a fever based on the body arises within his body, or there is sluggishness in his awareness, or his mind becomes scattered externally. He should then direct his mind to any inspiring theme. As his mind is directed to any inspiring theme, delight arises within him. In one who feels delight, rapture arises.

In one whose mind is enraptured, the body grows calm. His body calm, he feels pleasure. As he feels pleasure, his mind grows concentrated.

He reflects, ‘I have attained the aim to which my mind was directed. Let me withdraw [my mind from the inspiring theme].’ He withdraws & engages neither in directed thought nor in evaluation. He discerns, ‘I am not thinking or evaluating. I am inwardly mindful & at ease.’

“This, Ananda, is development based on directing.

And what is development based on not directing? A monk, when not directing his mind to external things, discerns, ‘My mind is not directed to external things. It is unconstricted [asankhitta] front & back—released & undirected. And then, I remain focused on the body in & of itself. I am ardent, alert, mindful, & at ease.’

“When not directing his mind to external things, he discerns, ‘My mind is not directed to external things. It is unconstricted front & back—released & undirected. And then, I remain focused on feelings… mind… mental qualities in & of themselves. I am ardent, alert, mindful, & at ease.’

“This, Ananda, is development based on not directing.

“Now, Ananda, I have taught you development based on directing and development based on not directing. What a teacher should do out of compassion for his disciples, seeking their welfare, that I have done for you. Over there are [places to sit at] the roots of trees. Over there are empty dwellings. Practice jhana, Ananda. Do not be heedless. Do not be remorseful in the future. That is our instruction to you all.” — SN 47:10