At first I thought this was a no-brainer, but then I gave it some thought?
Yes traditional books kill trees Unless done on recycled paper. They themselves can be recycled, fairly easily. The easiest way is to donate them to a library.
E-books don't use paper, but there are still issues with production, and and energy used just the read them. Also let's not forget the the amazing rate at which equipment becomes obsolete in out world. How recyclable are these units going to be?
I do use both, but I will keep my e-book reading to my laptop and not go and buy another piece of equipment just for that feature.