All Problems and Solutions - A set of solutions (not yet complete - 2018-05-27) to the problems set in the textbook, maintained by me, including even-numbered questions. Includes links to the PDFs detailing the exercises.
Applied Cryptography (Second Edition) by Bruce Schneier - I personally found this book fairly impenetrable, maybe try The Handbook of Applied Cryptography unless you have a strong mathematics background
American Cryptogram Association Magazine - Mentioned in the books “The Codebreakers” and “Crypto”. Members can join from the US or overseas to receive a bi-monthy magazine of cryptograms and their solutions. Those interested in classical cryptanalysis will likely want to join this organisation to sharpen their skills