There’s a free book available at http://www.smartbearsoftware.com/codecollab-code-review-book.php
I just finished reading it a few weeks ago. It’s a good read. It provides some good insigths about the different kinds of Code Review a company could use, but besides that, it also provides some good info that can make developers think a little more about how they write code so to make it easier for whoever is reading it to understand (which could be either another person acting as a reviewer, or even the author of the code him or herself having to maintain the code).
Claudio Lassala's Blog 