I'm looking for difficult problems on multivariable analysis: roughly the material covered in Spivak's Caculus on Manifolds, but with harder (and more) problems. What are some good textbook or problem books?
Some other books I've looked at (so please don't include these in your answer): Baby Rudin, Munkres' Analysis on Manifolds. The problem with these is that problems are mostly basic (Rudin) or too few in number (Munkres).