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
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?)