PineWikiBooks on reserve at the EngineeringAndAppliedSciencesLibrary.
1. Course textbook
Kenneth H. Rosen, Discrete Mathematics and Its Applications, Sixth Edition, McGraw-Hill, 2006. ISBN 0073312711. QA39.3 R67X 2007 (LC).
2. Other discrete math textbooks
Norman L. Biggs, Discrete Mathematics, second edition. Oxford University Press, 2002. ISBN 0 19 850717 8. QA76.9 M35 B54 2002.
Ronald L. Graham, Donald E. Knuth, and Oren Patashnik. Concrete Mathematics: A Foundation for Computer Science. Addison-Wesley, 1989. QA39.2 G733X 1989.
A survey of mathematics likely to come up in advanced theory work with an emphasis on enumerative combinatorics. Much of this material is also scattered across KnuthSeries, but it's more accessible here.
3. Proofs and problem-solving
George Polya. How to Solve It: A New Aspect of Mathematical Method. Princeton University Press, 1973. QA11 P65 1971
Daniel Solow. How to Read and Do Proofs: An Introduction to Mathematical Thought Processes, 4th edition. Wiley, 2005. QA9.54.S65X 2005 (LC).