There's one by Dr. Ruth (Dr. Ruth Westheimer), too, that was recommended on another thread for boys. I haven't had a chance to check it out yet, so I can't lend personal recommendations, but you might look for that one, too!
We loved "It's So Amazing". I gave it to my son right after he turned 8 in follow-up to a discussion we had. It hits all the mechanics in a fun way, and also has information on alternative families, masturbation, etc.