This book here is a little different than a standard Codex. This book was put out during the third edition of Warhammer 40k, and was originally intended to be used as a guideline for additional Space Marine armies that did not warrant a whole codex.
That time has come and gone, however, and what we are left with is an excellent book containing tons of background material and fluff that you wouldn\'t be able to find unless you scraped up all the second edition or prior rules. (Talking 10-15 years ago here)
This book is illustrated in full color and has detailed information on everything from the making of a Space Marine to some of the variant chapters many people have never heard of. (White Scars, Flesh Tearers, etc.) Chaos fans, don\'t miss out either, Iron Warriors, Emperor\'s Children, and more inside as well.
All in all, a great read, and while the rules themselves might not be current, the background story is, and is done in great detail here.