The Index Astartes books from
GW. You should be able to get them from amazon, ebay, etc ...
Each book (there are 4) is basically a collection of articles that were once published in the White Dwarf. They explain stuff like recruitment and creation of Space Marines, and discuss individual Chapters or specific roles in them. Loads of interesting background.
What you want is probably in the first book:
http://wh40k.lexicanum.com/wiki/Index_Astartes#Index_Astartes
They also avoid the "novel distortion" - since
40k does not actually have a "canon",
BL writers tend to just make a lot of stuff up as they go, at times clearly contradicting Codex fluff. Also, Index Astartes doesn't "hide" its fluff within a story.