Personally, I find the source code to be an invaluable reference for working with mysql.
As for a book, I bought one of my guys, the MySQL Administrator Guide,
http://www.amazon.com/exec/obidos/tg...l/-/0672326345 (affiliate code stripped off)
It was one of the few that went indepth into a few issues that were quite important (indexes & joins), but, might be a tough read and perhaps not really for general use/consumption.
There was another one that I saw at the bookstore, Beginning Mysql from Novice to Professional. I flipped through the pages, it was a much more cursory overview, and seemed written well.
I used to be a fan of the Zoo books (O'Reilly), but, lately, their content quality seems to have gone downhill.
My personal preference is always a reference style book, some people prefer tutorial type books. If I get a chance this weekend, I'll go to the bookstore and read through a few.