Please note that Best 40k Books II is currently active over in Fluff. Best 40k Books I will remain here for archive purposes.
FerociousBeast has created an index for this thread, which details where reviews/commentary on specific books/series can be found within the thread itself. It can be found in this post.
OK guy's, this is the last (I hope) thread about 40k literature. ionfish said he'll sticky it. Here are my reviews of the best 40k books, feel free to criticize or query as you feel appropriate.
For simplicity I'll post series together.
Crossfire/Legacy. Far lower on action than most 40k books and a lot more about the politics and societies of the Imperium.
Seen through the eyes of an Arbites officer (Shira Calpurnia) Crossfire takes a route through the twisted intrigues of Hydraphuran life. Well paced and thought out, it gives a good look at the Imperial aristocracy and some lesser known sides of the universe...
Legacy is less well paced than Crossfire but with some unprecedentedly cool action.
Gaunt's Ghosts. 9 books and counting! All of them are pretty good but the first two (First and Only, Ghostmaker) are split into sub-chapters which make it tricky to keep a handle on what is going on.
The Space Wolf series is, as you would expect a pretty ood look at the Space Wolf chapter through the eyes of Ragnar Blackmane. A good series but the emphasis on action gets a little samey at times. Space Wolf is probably the best of ths series for the look at the initiation of Space Marines and Fenrisian life.
For the Emperor/Caves of Ice. Commissar Cain is decidedly unusual in his methods, but it get's the job done and leaves a smile on your face. Told through his own eyes it combines humour and action (and the attempts to avoid it) in a lightweight but fun package.
The Inquisition War trilogy. Possibly the darkest and craziest books the Black library (and boxtree) have ever printed. it follows the adventures of Inquisitor Jaq Draco through the highs and lows of the Imperium via Stalinvast, the Emperor's throne room and the webway to name buy the ones that won't spoil anything. The tone grows steadily madder through the series which makes it a little hard to stick occasionally but if you can last it it will reward you well.