I see that you have already made your choice, but here's my $0.02. If it's not a far ride to the library, I'd go with the backpack, as they are generally easier to carry around than a Panier, once you're at the library. If you already have a few backpacks, why not give one a try to see if you like it before you buy a new anything? Granted, I'm all for getting new bike stuff, so I would be tempted to buy a new bag too.