Clay Jannon, a web designer, accidentally ends up in a round-the-clock bookstore, where he starts working as a clerk. He discovers that the shop provides mysterious books in exchange for other mysterious volumes, and he begins unraveling the secrets hidden by Mr. Penumbra. Coders, librarians, archaeologists, hackers, and other specialists help him as he deciphers the codes. The novel “Mr. Penumbra’s 24-Hour Bookstore” became a bestseller and was published in more than thirty countries.