infiniteskills.com has educational videos, and recently developed a new one about cryptology. It's not a book, but it will teach you plenty. Also, Hacking The Art Of Exploitation had a chapter on encryption, but I havn't read it thoroughly enough to recommend it or not. Hacker Linux Uncovered has a nice intro to hashing, I though it was very informative.
I was researching exploitation, and am interested in a potential career as a professional exploit developer or bug hunter. However, with all the security mechanisms in memory, I want to know whether this is even worth pursuing, or are vulnerabilities still likely?