One primary difference is that libraries have real scarcity; they only have so many copies of any given book, only so many books in total, and a limited timeframe for borrowing.  That's most important for new releases, where the number of people who want to read the book far outweighs the number of copies the library might have.  It also holds to a lesser extent for people who want to read books the library is less likely to have (for example, if you have a favorite author, your local library might not have all their books), and for reference books, where you don't want to be constrained to a particular timeframe.<p>If libraries had unlimited copies of all books in print and you could borrow them instantly whenever you needed them, most of the main motivations for buying books would disappear, and the book publishing industry would operate very differently than it does today (whether or not that would be better, by providing more access to information, or worse, by removing the incentive to create, is open for debate).<p>So while in some sense downloading a book and getting it from the library might feel the same, in that you're reading the book without having to pay for it, the reality is different because all of those constraints have been removed.<p>As with things like music and movies, the primary problem here is that the physical medium imposed real constraints that are a big part of how and why the current business model for publishing has functioned.  Just because it's possible to remove those constraints doesn't mean it's necessarily the ethical or pragmatic thing to do.<p>For the case of libraries, I think it's very much up in the air what the future will look like.  You could imagine imposing the same constraints on digital copies via DRM, or imposing those contraints only for new releases, or charging for digital access but leaving physical access as free, or simply moving to some sort compulsory licensing model.  But I don't think that digital equivalent exists yet, and I think it'll be a long, difficult road to ironing those questions out.