Well judging by your avatar have yoe tried "Lords of Night"?. In that books case, though it wasn't bad, I'm not very fond of the "misunderstood" traitor legion stories.
Here's the Black Library site
http://www.blacklibrary.com/default.asp
The various books range widely in quality. I haven't read them all but I do hang out at a GW forum so I have gathered information on most of them. Let's start with the page I gave you. Let's see....
Souldrinkers. Well I bought the first 2. I wouldn't call the quality bad, however I personally dislike the chapter (As I said I don't have much sympathy for traitors)
Deathworld. A friend at the store showed it to me...for about 2 seconds. I'm not interested in Catachans (self styled Rambo, or should that be Sly Marbo, ripoffs)
I don't read Necromunda, and not much fantasy either, though Felix and Gotrek books are considered worthwhile.
DO read Horus Heresy books. The first three are by BL's top writers. The next one I intend to get, hopefully it will be good, though as it's written by the guy who wrote the Blood Angels pair I have my doubts. The bloke in question is from what I consider the "D and D" school of churning out books. Hackneyed novels with little basis in the background as if they don't read the manual.
Ask me about any of the books and I can probbaly explain it. Ravenor is highly praised. I've never read it myself.
Warriors of Ultramar is my favourite, not so much for the whole book, which is good enough, as for the fact I collect Mortifactors so I'm a bit biased. Off the top of my head "Deamon World" was a great book. Stay well away from "15 Hours" DandD hack crap that goes nowhere. Guants ghosts are good. More like ww2 stuff moved into M40, but not bad. Abnett came up with a new way for representing Chaos.
I have a few original Inqusition trilogy books and a rare "Space Marine" paperback. They however are horribly out of date Background wise, but highly prized. The Inquisition Trilogy was released awhile back as an omnibus.