That depends on what you want to do with it. I found Katz & Lindell's Modern Cryptography, a textbook written by active research cryptographers, quite good. Th

That depends on what you want to do with it. I found Katz & Lindell's Modern Cryptography, a textbook written by active research cryptographers, quite good. The classical "applied" work is Schneier's Applied Cryptography", but note there are a lot of issues with that work. His newer book, "Cryptography Engineering", is supposedly better. Goldreich's Foundations of Cryptography is quite solid, but requires quite a bit of background and is very theoretical.