Jeff Hoffstein, Jill Pipher, Joseph H. Silverman, An Introduction to Mathematical Cryptography, Undergraduate Texts in Mathematics, Springer, 2008.  

Available as e-book:        

Richard A. Mollin, An Introduction to Cryptography, Chapman & Hall/CRC, 2001.

Second edition (2006)- Available as e-book:

Richard A. Mollin, RSA and Public-key Cryptography, Chapman & Hall/CRC, 2003.

Available as e-book:

Wade Trappe and Lawrence C. Washington, Introduction to Cryptography with Coding Theory, 2002.   Second edition (2006)

Lawrence C. Washington, Elliptic Curves: Number Theory and Cryptography, 2nd edition (or more recent if available), Chapman & Hall/CRC, 2003.  Second edition (2008)