Michael Hart of gutenberg project fame estimates mankind has produced so far about 2 millions books (all languages), about 1 millions are lost. If we assume 400 pages per book on average, with each page having 50 lines of 50 characters and one byte per character, that's 1 megabyte per book and 1 terabyte for all of mankind still available books. With compression of one bit per byte that's 125 gigabyte, a hard drive of this size currently cost less than 50 euros TTC in France.

Current "bistable" display technology use no power for static display.

So when we ask what book I want to take, just remember that if we have to choose it comes from intellectual property laws, not from physics or technology limits.

The whole wikipedia text in several language already fits on your cellphone or PDA MicroSD card (assuming you have an open software cellphone or PDA).