Angels of Darkness is the third and last book in the Dark Angels Space Marines story.
The first thing you notice while reading the book is the different author. We're still told two stories (one of a Fallen and one of an Interrogator-Chaplain, both Dark Angels) as in the second book, but although both are told in present tense, one happens before.
Also, the difference in writing is noticeable, not a bad thing, just you feel different than with the previous books. Probably having completely new characters boosts this feeling too.
This book deeps a bit more into how a Space Marine thinks. Questioning of oaths, duty, preservation of lives, and secrets, dark secrets between the chapter.
The combats are nicely described (and there are a bunch), although killable this Space Marines are real badasses, and in general the setting is really good (probably the best of the three books).
As usual it ends with a few unanswered questions, but it at least makes some really interesting revelations of the Dark Angels. One warning, though. Without spoiling anything, Astelan's tale has been confirmed to be subjective and not the reality.
If you enjoyed the other too books, you must read this one and finish the trilogy.