The ultimate problem book for math/engrg/science majors. The problems are divided into categories, some in math, some in physics, others in engrg, and they are deep. It takes a lot of thought, a few reference books, and a lot of scratch paper to solve these.
The copy I went through came from the library. It is an expensive book, and I am buying a copy for my bookshelf. I want to go back into it and rework a problem now & then.