I bought this book, Descent of Angels, because I saw it was about the Dark Angels (my Space Marine's army chapter). It tells us the story about the origins of the chapter and it's Primarch, all on their home planet Caliban.
The only problem is that the book just explains that. Almost no first steps of the Dark Angels as Space Marines, just the beginnings as knights... Which is good if you don't expect more (like I did), but might be not enough if you also expected their first steps as Space Marines.
Later I found there are two more books that continue the story so it's not a big deal, but be warned, it leaves you with just a glimpse of the real story.
It is well written, although quite typical tale of the creation of some heroes(just with a Warhammer 40.000 background, and even with that, most of the story is a fantasy knights order with monsters one), decorated with a few plot twists basic and predictable. Nothing impressive but as far as you know the terms of the Warhammer 40k universe you will read it fast (I read it in three days).
Overall, a just decent book, more interesting if you want to expand your knowledge of this Space Marines. Else is of little interest as talks more about medieval-like orders than space marines or alien races. An intro for the following books.