My son is about to be potty trained and to help me a long my friend purchased the book What to Expect When You Use the Potty written by Heidi Murkoff (co-author of What to Expect When You're Expecting) as a gift.
While this book is illustrated beautifully and there are lots of suggestions on how to help your child use the toilet this book may be a little confusing for some toddlers. This book is set up for a parent to read to their child to help them understand exactly what happens when they need to use the toilet to the importance of hygiene. The problem that I found with this book is that my son's vocabulary is not large enough to understand many of the words or even the concepts. For right now he just likes to look at the pictures.
I believe that this book would be perfect for an older toddler who is not yet potty trained. But for a child between 18 and 30 months the "story" is too advanced.