- Elementary Number Theory- Kenneth Rosen: I thought this was a good introduction to number theory and computational aspects, including some cryptography. Later chapters became more focused on theorems and proofs, without really providing enough by themselves for the reader to attempt a lot of the homework problems.
- Applied Combinatorics- Alan Tucker: I continue to use this book as a reference. It's great if you have a basic grasp of graph theory or combinatorics and want to go into some more detail. It's definitely a strong undergraduate introduction to both topics, and it really leans more on the computer science end than the math end (at least, that's how I felt).
- Cryptography: Theory and Practice- Douglas Stinson: I'm using this book in my Crypto class this coming semester. I've glanced through it, and it is definitely an applications book rather than strictly theory. It also isn't necessarily as friendly as the early chapters in Rosen's Number Theory book. One thing I do like is that Stinson talks about the cryptographic schemas in terms of groups, rather than simply limiting to the canonical group Z mod p.
Other good books include Donald Knuth's The Art of Computer Programming, and Sedgewick's book on Algorithms.