Jeff Hoffstein, Jill Pipher, Joseph H. Silverman, An Introduction to Mathematical Cryptography, Undergraduate Texts in Mathematics, Springer, 2008.
Available as e-book: https://link.springer.com/book/10.1007%2F978-1-4939-1711-2
Richard A. Mollin, An Introduction to Cryptography, Chapman & Hall/CRC, 2001.
Second edition (2006)- Available as e-book: https://www.taylorfrancis.com/books/9781420011241
Richard A. Mollin, RSA and Public-key Cryptography, Chapman & Hall/CRC, 2003.
Available as e-book: https://www.taylorfrancis.com/books/9781420011241
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)