This is a recurring question on the board. I shift more toward e-reader as time passes:
- fiction: electronic
- most non-fiction: electronic
- reference & how-to: paper
- news: both
- some non-fiction: paper (anything I want to modify with underlines, marginalia, and sticky bookmarks)
- some classics: if I ever get around to reading Homer or Gilgamesh, for example, I'll probably want paper