I got an L.L. Bean backpack ... prob'ly 20 years ago. WOrks peachily. Plastic grocery sacks aroiund my books if it's going to rain on me (which it basicallyh never does, but that's another story ) and I"m good to go.
(That is, of course, if I have to ride my Trek or Gazelle... normally I'm on the Xtracycle and big stuff goes in the saddlebags and I wear a camelbak for the lights on it.)