Paging is a good way to see how the nuts and bolts of a library works...and to be the person on whom the library's continued smooth operation hinges. Because if you don't get books back into their place on time, then I end up not being able to give them to people who want them...and that's very sad-face inducing.

Give your boss time to let up on you - the assurance of being able to put stuff back correctly is important, and now that you're demonstrating competence, they'll hopefully move the Eye of Sauron elsewhere soon.