The cryptology literature and other sources mention several dozen books that are entirely or partially encrypted. However, a systematic treatise about this issue has never been made. This article aims to close this gap. The author has assembled a list of encrypted books that contains 41 entries, so far. Based on this list, an overview on encrypted books is given. Encrypted books are classified according to criteria like purpose, solved/unsolved, encryption method, and creation time. Numerous open research questions are identified. Finally, it is a goal of this article to ask readers for additional information about the books on the list and for encrypted books that are still missing.