I collect dictionaries (yes, I know that's weird, but I love to see the way word meanings change -- someday I hope to have a copy of the OED
)
I make it a point to keep paper copies of series that I particularly like (Anne McCaffrey's Pern books, for instance).
Beyond that, I recently decided that the "keep every book I ever bought" strategy was consuming my living space, and started getting rid of the ones I figure I never want to read again. I'm also trying to acquire books in e-format as much as I can now, but I'm sticking to 'unlocked' formats that my kids can inherit someday.
