What are in-jokes? “In-jokes,” or “inside jokes,” are jokes whose humor is clear only to members of a certain group, and not the general audience. For example, in-jokes in textbooks will often be understood mostly by people fluent in the subject matter. Other cases include jokes targeted at an even smaller population, even as small as the author's family.

In computer science books, many in-jokes deal with self reference; others deal with loops, and others still are about repeated ideas. But of course, there are many other kinds, including obscure references to other works of art.

This list includes not only in-jokes per-se, but also jokes that hide in a book's appendices, in the index, in the “solutions” sections of textbooks, and other infrequently-read parts of books.

In-jokes count as such only when they aappear in otherwise serious books -- not jokes in generally comical books. Certain books, like Gödel, Escher, Bach or The Cyberiad, are loaded with jokes -- but mostly, these are obvious puns, word-games, and suchlike rather than in-jokes. So most jokes from GEB won't qualify. (Two examples that do qualify are listed here.)