My daughter also potty trained before she was 2, and I found it difficult to locate potty books aimed to really young kids. One that we both LOVED was an Elmo board book that had a press-the-button side panel. Each button had a bathroom noise, like flushing or bubbles, or a silly Elmo phrase. I think I bought it at Walmart. The thing is, ANY book with noises is bound to be more interesting to this age group, so look for a character your son connects with. Also good are books that rhyme - they hold a small child's attention better. Really, though, you may just want to go straight to a bookstore and browse - you'll have a better idea of how long a story your son will listen, and whether the pictures and storyline will interest him.
No matter what, you'll probably want at least 2 potty books - I never did find an age appropriate book that truly covered EVERY SINGLE ASPECT of potty-training (realize you have to go, HURRY, pull down pants, sit, wait, use TP, pull up pants, let Mom dump potty, flush, wash, don't worry about accidents - there was always SOMETHING missing!). Plus, you'll read these books, oh, about 1,000 times in the next few months, so variety can be nice!
Good luck and congratulations!
-K.