In addition to Concrete Mathematics by Knuth, which contains a lot of Mathematical tools and techniques useful in areas of Computer Science, I'd recommend the following to learn how good problem solvers approach Mathematics:
1. The Art and Craft of Problem Solving by Paul Zeitz.
2. How to Solve It by George Polya.
I'd also recommend learning any of the competition Mathematics topics like number theory, combinatorics or geometry as base material to improve your problem solving skills. I have also found the Schaum's series on Combinatorics and Graph Theory concise and useful.
1. The Art and Craft of Problem Solving by Paul Zeitz.
2. How to Solve It by George Polya.
I'd also recommend learning any of the competition Mathematics topics like number theory, combinatorics or geometry as base material to improve your problem solving skills. I have also found the Schaum's series on Combinatorics and Graph Theory concise and useful.