I just finished reading the book Angels of Darkness by Gav Thorpe.
I was amazed to see that it was a better read than the Horus Heresy Novel Descent of Angels.
If you have a chance to pick this book up I highly recommend it. The novel takes 2 parts, one following Interorgator-Chaplain Boreas as he works on getting Astelan (An original Chapter Comd of the Dark Angels pre-legion uniting with Lion El'Jonson) to confuse his traitor ways. The other part is Boreas dealing with his own issues and a sneaking feeling that Astelan may be right. I won't spoil the book as it is a great read and reveals more about the Dark Angels than the Horus Heresy book, I would also argue that this book is a better fit in the series than Desecnt of Angels was. If you like the Horus Heresy novels this book fits in perfectly.