Brookshear, J. G. Theory of Computation: Formal Languages, Automata, and Complexity. Benjamin/Cummings, Redwood City, CA, 1989. Shows the relationships of the theoretical aspects of computer science to set theory and the integers.
Hardy, G. H. and Wright, E. M. An Introduction to the Theory of Numbers. 6th ed. Oxford University Press, New York, 2008.
Niven, I. and Zuckerman, H. S. An Introduction to the Theory of Numbers. 5th ed. Wiley, New York, 1991.
Vanden Eynden, C. Elementary Number Theory. 2nd ed. Waveland Press, Long Grove IL, 2001.