"Mr. Penumbra's 24-hour Bookstore" by Robin Sloan

What an amusing book. It tells the tale of a young man who gets a job in a mysterious bookstore. It's open 24/7 but almost no-one ever stops in. The shop is filled with ancient books which are hardly ever checked out. Members of a secret society borrow books occasionally, to try to decode the texts they contain. As the book progresses we find out it's all part of a group of scholars who've been working on decoding the books for over 500 years. I won't give away any more of the plot, but I enjoyed it quite a bit.