If you are already familiar with the basics of codebreaking, or even if you are new to it, then the BOSS Cryptanalyst handbook would be a good place to sharpen your skills for the competition. It starts with the familiar Caesar cipher and covers a number of topics with descriptions of the classic ciphers and some tips on how to crack them.
To give you flavour of what you can find there, here is the contents page. The book is free to download as a pdf and is just one in a series of useful guides you can find in our Library.