Paper books are fucking HEAVY and bulky. They're only practical if you don't ever need to take more than one or two anywhere. If you gotta be on the move, the extra equipment to manage the battery is much lighter than the books you would need to carry otherwise.
Another advantage to an e-reader is that I can remove the battery and it's completely waterproof, as long I completely dry it before the battery goes back in, it can get as wet as I need it to, no problem. Books don't have that option.