The amount of space ebooks take up is generally negligible. The complete works of
Shakespeare, in epub format, are 2.2 MB on Project Gutenberg. That's about the size of a picture from my digital camera. I think my whole library of a couple of thousand ebooks takes up less than half of a 4 GB chip. Space really isn't an issue.
As for getting books conveniently to your users, follow Manichean's advice (always a good idea anyway) and check out the calibre content server.