Ever since "The Ultra Secret, Winterbotham"
https://books.google.co.uk/books/abo...er&redir_esc=y came out I was (am) fascinated by the whole thing. Some of the maths is a bit tricky, worth getting to grips with. And some of it is still secret. There is an American paper "A general report on Tunny" here
http://www.alanturing.net/turing_arc...portindex.html Turing didn't have a lot or anything to do with Tunny though.