I have a long-standing interest in Cryptography, particularly the kind that existed before computers came around.
I've spent periods obsessed with the Voynich Manuscript, and have written a few different software programs for decoding cryptograms of various kinds, as well as constructing crossword puzzles, which is a related kind of activity. My love of cryptograms inspired my puzzle Krypto Kakuro
A good first book on this subject is David Kahn's "The Codebreakers". My favorite book about the goings-on at Bletchley Park during World War II are "The Codebreakers" by Hinsley and Stripp, and Hodges biography of Alan Turing.