The Potty Training Answer Book, by Karen Deerwester.
My two older boys were potty trained around the age of three - give or take a few months - using the Toilet Training in Less Than A Day method (works great as long as you don't actually expect it to take only one day). So imagine my surprise when, and the age of 19 months, my third son developed an intense interest in using the potty. He started out wanting to read Once Upon a Potty over, and over, and over, again; then when I got out the little potty seat, wanted to sit on it. Maybe it is the fact that he is the first child to wear cloth diapers in our family, but he has already made the mental connection between what he feels in his body and the pee pee coming out. He has limited bladder control, meaning he can wait just a few seconds, time enough to get to the potty and for me to get his diaper off. I wasn't quite prepared to do the full-scale training just yet, so I'm not pushing it hard at this point.
The Potty Training Answer Book really does answer practically every question one might have on the subject. It doesn't espouse any particular method, but does discuss different approaches to training (or "potty learning", as some prefer to call it). Therefore, it is a good companion to "method" books like Less Than A Day, or as a reference guide if you want to develop a "Personal Potty Plan". The most helpful parts of the book were the discussion on child temperaments, and how training approaches should be customized for each temperament; and the list of resources in the back such as books, videos, games, songs, websites, and products.
It's also the sort of book that you can read all the way through, or turn to a particular section. The beginning of each chapter lists all the "questions" answered in that chapter, which make the answers easy to find. On a completely coincidental note, the author, Karen Deerwester, is the parenting expert for Bluesuitmom.com, a website started by Maria T. Bailey whose book, Marketing to Moms, I just read and reviewed.