It's odd, but I find myself deleting books after I've read them, secure in the knowledge I can re-download them later. I have something like 75 books on the thing, some purchased, some freely available in the public domain, and some out-of-print-but-not-freely-available. But I tend not to keep them around forever, because I think that if I ever get near the 1500-book limit on mine, it will be too unwieldy.

Some books, however, I won't ever delete, because I reread them often.