Book cipher
Each word or letter is replaced by a pointer — page, line, word — into a book that both sender and recipient own.
The key is the book itself, which is both the appeal and the flaw. An ordinary volume on a shelf raises no suspicion, but whoever identifies the edition can read everything. In 1780 Benedict Arnold and the British major John André used Blackstone's Commentaries this way while Arnold arranged to hand West Point to the British.
The most famous case is the Beale ciphers: three number sequences published in the 1880s that supposedly locate a treasure buried in Virginia. The second turned out to be keyed to the Declaration of Independence. The other two remain unsolved, which means either the key book is unknown or someone played a very long joke on posterity.