We let ds choose. If they like a book, I don't see any reason to remove it as repetition is how they learn.
We rotate books every 2-3 weeks at the library. Our choice of books really has to do with what ds is interested in at the time and things we want him to learn (everything from letters, shapes, numbers, to manners and body parts).
We did find that when he was no longer listening to a book or flipped pages quickly to finish a book, that it was time for more 'content'. With that generally comes paper pages as opposed to board books.
I think it is just a natural progression, which we encountered at about 2.5yo.