1. Varian's grad-level Microeconomic Analysis. Offers a bit more intuition for the 601 stuff.
2. Varian's Intermediate text, also known as `baby Varian.' Useful for intuition as well.
3. Lones recommends Gibbons's Game Theory for Applied Economists for an accessable game theory text. We do, too.
4. Krep's A Course in Microeconomic Theory is particular recommended for 603/604 material.
5. Nicholson's text is also highly recommended.
6. The Structure of Economics: A Mathematical Analysis by Eugene Silberberg, Wing Suen, Wing Chuen Suen gets the following recommendation:

   ...extremely helpful and enlightening and fills in the gaps of Mas-Collel well, esp on the nuts and bolts of the actual work and arriving at results rather than seemingly arriving cryptically at some results. And they give some good intuition, as well as devoting an entire chapter to the envelope theorem.

• Lones recommends How to Solve It by George Polya which is general heuristics for solving general problems.
• On proofs: How to Prove It : A Structured Approach by Daniel J. Velleman. Recommended for general principles of proof techniques.
• Also on proofs: How to Read and Do Proofs : An Introduction to Mathematical Thought Processes by Daniel Solow.

Note: Amazon links above are just for reference. we at the GES have no particular preference for amazon over other booksellers.