After Once Upon a Potty -- Boy, my favorite picture book on potty training. Ducky has a friend come over who can use the potty like a big kid, and Ducky (gender is unspecified) begins to notice how wet and uncomfortable a diaper is and soon is taking a turn on the potty.
Since this is every baby's favorite game, throwing everything "overboard" for mommy and daddy to pick up over and over again, the book is definitely one a baby can understand. Although, I'm afraid it encourages more of throwing items "overboard" for fun.