Niche topic for a maths personal statement

Hi all!! I am almost finished with my first draft of my personal statement and im just missing one section where I want to show my love of maths in its application. I really love cryptography and coding theory but I can't find a proper book on the topic rather than just a work book, so Ive given up on that and ill literally be interested in anything if theres a good enough book. so pls can someone tell me a good book to read on a niche topic of higher education mathematics.