If you're going run this kind of unsanctioned online library, the bare minimum requirement is that you don't lend out more copies than you own. You can't copy one book and lend it out to two people at the same time, and you can't lend out the copy without holding the original in escrow. I have seen it said that Internet Archive wasn't doing this, or else they could reasonably argue they were acting in line with existing case law (that copying media you own for backup purposes is permitted) and the spirit of the law.