Other than the Space Wolves series, one of my favorite Black Library books was a Sisters of Battle novel called "Faith & Fire". It was released back in 2006, and is now being re-released along with it's sequel, "Hammer & Anvil". The first book I enjoyed because it had believable characters, with actual human emotions. It also had enough back story to weave it into the history and mystery of the 40K universe, touching on subjects that give you those "a-ha" moments like the Horus Heresy novels do.
The sequel is focusing on the Sisters of Battle returning to the fabled Sanctuary 101, the first Necron siting in the modern 40K universe and the site of the slaughtering of the Sisters of Battle stationed there. I'm anxious to read this book and see how they have fleshed out that story. I'll give GW credit, they seem to be doing a great job of tying in all there product lines with their latest releases. A lot of folks have criticized them for the way they handle their rumors and new releases. For me however, it's a nice surprise to find little gems like this in my Google Reader every now and then!