Techniques for Proving the Correctness of Algorithms
Proving algorithm correctness by Rod Howell (online textbook on algorithms)
Discrete mathematics textbooks
Fundamentals of Discrete Math for Computer Science: A Problem-Solving Primer, Stephenson Jenkyns
Available for free at http://link.springer.com/book/10.1007%2F978-1-4471-4069-6 from a UCSD internet connection.
Mathematics for Computer Science by Lehman, Leighton and Meyer