Are LDS church manuals considered to be the words of God?

First a definition of "word of god":

Doctrine and Covenants 68:4

4 And whatsoever they shall speak when moved upon by the Holy Ghost shall be scripture, shall be the will of the Lord, shall be the mind of the Lord, shall be the word of the Lord, shall be the voice of the Lord, and the power of God unto salvation.

So, in that sense, everything said by inspiration is the word of God. Should be obvious, but is not.

A manual is a manual, not more. Manuals cite the word of God (scriptures, prophets, etc), and they are approved by our leaders, but manuals still stay just manuals. They have the authority that they get from my last sentence. But it's not like there will be a revolution if some revelation comes that renders a particular doctrine taught in a manual invalid. It just means the offered interpretation was wrong. I wouldn't be surprised if such stuff happens/happened. The first members surely understood some things differently than us. Manuals about Christ in the spirit world surely read different things before vs. after the revelation of D&C 138.

As to books written by apostles and such, they are pretty good, and you have a good chance they are "right" and "inspired" (and thus God's word), but remember that official doctrine should be official. That means General Conference - official. The reason being that in this setting, the prophet could intervene if false teachings are spread (and this has happened, people tell me, when McConkey taught about why blacks didn't receive the priesthood).

The handbooks are the rules about how we do stuff in the church (like conducting meetings, tralala). We should abide by them, but it's not like we can't sometimes deviate if necessary. One of the reasons they are not generally distributed to everyone, is to not undermine local leader authority by members saying "Well that didn't go according to handbook". The handbook 2 states also that:

Occasionally the information in these handbooks will be updated or supplemented through letters, notices, and other communication from the First Presidency, Quorum of the Twelve, and Presiding Bishopric. When this occurs, leaders should note the changes in their copies of the handbooks. Leaders should keep handbooks and these supplementary materials together.

So yeah, word of God but not unchanging. Just follow it, and be prepared for new instructions.

As to Preach My Gospel, it is really, really good. It probably counts as "Word of God", but let's face it: It is a well-written, inspired manual that will bring you forward in your life. Nothing you make into canon, but... I agree with Pres. Packer. Study

I named the bad word - canon. Because, you know, we have such a thing, and we have it for a reason. This is the stuff really recognized as "The Doctrine"(tm). For all the other stuff, it sucks if it turns out we were wrong and have to change the manuals because of new revelation. Handbooks can change because. But the canon will only be added to. If the canon and some book or manual disagree, canon wins. (NOT our interpretation of the canon!)

If someone has a better answer, or some better sourced answer, please let us hear. And note that answers on StackExchange are not official doctrine ;-)

(see also Matts comment, which points to What works do the LDS Church recognize as Scripture?)

No, church manuals are not considered 100% church doctrine, if you define doctrine to mean the word of God. They are very inspired teachings and they contain what is being taught in the church, because they are used to teach. They are well considered, but they might not be fully free of personal opinions or even errors.

In October 2012 the LDS apostle Neil L. Andersen gave a definition of what constitutes doctrine of the Church of Jesus Christ of Latter-Day Saints in a talk at General Conference:

A few question their faith when they find a statement made by a Church leader decades ago that seems incongruent with our doctrine. There is an important principle that governs the doctrine of the Church. The doctrine is taught by all 15 members of the First Presidency and Quorum of the Twelve. It is not hidden in an obscure paragraph of one talk. True principles are taught frequently and by many. Our doctrine is not difficult to find.

(Emphasis added.) Similarly, apostle M. Russell Ballard explains what the voice of the Lord is in a General Conference talk in October 2014:

[...] when the First Presidency and the Quorum of the Twelve speak with a united voice, it is the voice of the Lord for that time. The Lord reminds us, ‘Whether by mine own voice or by the voice of my servants, it is the same’ [D&C 1:38].

(Emphasis added.) Therefore, even a talk from the president of the Church does not necessarily constitute doctrine. However, the official canon of scripture (Bible, Book of Mormon, Doctrine and Covenants and Pearl of Great Price) and all things that have been given by the First Presidency and the Quorum of the Twelve Apostles are considered doctrine. This includes The Living Christ and The Family: A Proclamation to the World. These things make up doctrine and are unchangeable eternal principles for the Church membership.

The apostles and prophets of the LDS Church are not considered infallible by its members. However, whenever a person speaks the words that are given him through the Holy Ghost, then this can be considered the word of the Lord and scripture according to D&C 68:4:

And whatsoever they shall speak when moved upon by the Holy Ghost shall be scripture, shall be the will of the Lord, shall be the mind of the Lord, shall be the word of the Lord, shall be the voice of the Lord, and the power of God unto salvation.

This can happen during a general conference talk as well as during a priesthood blessing or just a missionary teaching someone.

Regarding Preach My Gospel: It's a very uplifting and inspiring manual about missionary work. But again, it is not considered scripture, even though it might be very close to it. Just as other Church manuals it is subject to change and improvements.