Books are usually around 1Mb.
How many people listen to music they've downloaded, vs read books they've downloaded? How willing are people to stockpile books they might want to read, versus music they might want to listen to?
I have a friend with more books on his hard drive than my county library system has on their shelves. In case he ever wants to read them. In case of nuclear war. In case of anything. He finds that about half of my new-read requests are fulfilled on the internet, via simple torrent sites which are 1-step removed from the usenet and IRC scene. Surprisingly, this hasn't affected his buying habits.
He told me to start with the Great Science Textbooks 2007 DVD library, which comes in 20 parts (the final one was a few months ago, when the Knowledge-Should-Be-Free-based releaser considered his quest finished and stopped compiling), and covers a lot more than physics and biology. Supplement that with a few science fiction library dumps, some programming stuff in areas you're interested in, and you're golden to read until you die.
Oh, and get Calibre.