What is The eighth jhana and how can i practice it?

This book may help with jhana: Mindfulness, Bliss and Beyond by Ajahn Brahm.

By mastering access concentration, the 1st jhana, the 2nd, the third, and so forth. You really can't just suddenly up and practice the eighth jhana (and that jhana specifically for some very technical reasons). There have been a host of books written on the topic. A lot of them (including the one mentioned in Dhammadhatu's answer) are pretty good. However, there is no substitute to working with a teacher. That's especially true as you first get started. I would recommend that you seek out a jhana teacher in your area or go on a meditation retreat. Simply googling "jhana retreat" will return quite a few places you could attend worldwide.

Theory
3 “‘The sphere of neither-perception-nor-non-perception, the sphere of neither-perception-nor-nonperception,’
it is said. What now is the sphere of neither-perception-nor-non-perception?

4 Then, avuso, this occurred to me:
‘Here, bhikshu, by completely transcending the sphere of nothingness, he enters and dwells in the
sphere of neither-perception-nor-non-perception.’8
Practice

5 Then, avuso, by completely transcending the sphere of nothingness, I entered and dwelled in the
sphere of neither-perception-nor-non-perception.
While I dwelled therein, avuso, perception and attention accompanied by sphere of nothingness
assailed me.

Realization
6 Then, avuso, the Blessed One approached me by means of psychic power and said this to me:
‘Moggallāna, Moggallāna! Do not be heedless, brahmin, regarding the sphere of neither-perceptionnor-non-perception:
steady your mind in the sphere of
neither-perception-nor-non-perception, nevasaññānāsaññ’āyatane cittaṁ saṇṭhapehi,
unify your mind in the sphere of
neither-perception-nor-non-perception, nevasaññānāsaññ’āyatane cittaṁ ekodiṁ karohi,
concentrate your mind in the sphere of
neither-perception-nor-non-perception, nevasaññānāsaññ’āyatane cittaṁ samādaha.’

7 Then, avuso, on another occasion, by completely transcending the sphere of nothingness, I entered
and dwelled in the sphere of neither-perception-nor-non-perception.
Spiritual friendship

8 Now, avuso, if one speaking rightly, were to say, ‘He is a disciple who attained to greatness of
direct knowledge with the help of the Teacher,’
it is of me that one speaking rightly would say, ‘He is a disciple who attained to greatness of direct
knowledge with the help of the Teacher.’”

N’eva,saññā,nâsaññ’āyatana Pañha Sutta

Before getting there you have to practice the 7 leading upto the 8th. For more reference on this see this answer.

Also I am not sure if you can reach this without a proper teacher. Many teachers do not teach this. Perhaps one teacher who might teach it is Venerable Pa-Auk Sayadaw of the Pa-Auk Forest Monastery. Perhaps Ajahn Brahm also might be teaching this according to another answer. You can inquire if one of these teachers or their assistants are willing to teach you.

Also not everyone can achieve this level of deep Jhana.

@ DeusIIXII, what you are referring to is the last of the four formless meditations that exist. The eight attainments (atthasmapatti) in meditation (attaining Adhicitta) broadly has two parts - namely, the first four rupa jhanas and the four arupa jhanas. These last four are higher stages of mental concentration – the formless states - enable one to gain supernormal powers.

Eventhough we see the mind as normally inter-dependent with body, there are levels of existence where only mental phenomena exist, with nothing whatever of rupa. The four 'formless' (arupa) meditative states are attainable from the fourth jhana onwards. They are:

The sphere of infinite space (Akasanañcayatana);
This is attained by transcending any cognition of rupa, by abandoning the metal image that was previously the object of concentration, and seeing that space is infinite.

The sphere of infinite consciousness (Viññaaañcayatana);
In this second state, the focus is on the consciousness that had been aware of infinite space.

The sphere of nothingness (Akiñcaññayatana);
In the third, this object (Viññaaañcayatana) is dropped, and the focus is on the apparent nothingness remaining.

The sphere of neither-cognition-nor-non-cognition (Nevasaññanâsaññayatana). In the fourth, this object (Akiñcaññayatana) is dropped and the mind is in an attenuated state where it is hardly functioning.

If you want to know these stages in more detail please read the following Suttas:

SD 24.11 _ Pathama Jhana Pañha Sutta or (Savitakka) Pathama Jhana Sutta. Progressing in the 1st dhyana.
SD24.12a _ Dutiya Jhana Pañha Sutta or (Avitakka) Dutiya Jhana Sutta. Progressing in 2nd dhyana.
SD 24.13 _ Tatiya Jhana Pañha Sutta or (Sukhena) Tatiya Jhana Sutta. Progressing in the 3rd dhyana.
SD 24.14 _ Catuttha Jhana Pañha Sutta or (Upekkhaka) Catuttha Jhana Sutta. How to progress in the 4th dhyana.
SD 24.15 _ Akasanañcayatana Pañha Sutta. Progressing in the sphere of infinite space.
SD 24.16 _ Viññaaañcayatana Pañha Sutta. Progressing in the sphere of infinite consciousness.
SD 24.17 _ Akiñcaññayatana Pañha Sutta. Progressing in the sphere of nothingness.
SD 24.18 _ Nevasaññanâsaññayatana Pañha Sutta. Progressing in the sphere of neither-perception-nor-non-perception.

You might find this book useful: "Practicing The Jhanas", by Tina Rasmussen and Stephen Snyder.

They were both students of Pa Auk Sayadaw and undertook a Samatha Meditation Course under his guidance. They write in the book how they achieved the 4 material jhanas and the 4 immaterial jhanas.