**Schaum's Outline series** is a great resource for studying for tests, learning, and keeping on your bookshelf for reference. I have about 20 of them. There is a book about everything and I never paid for any one book (used, like new) more than $7 with shipping on ebay. The best ones are the Numerical Methods, Electric Circuits, Feedback Control Theory, Analog and Digital Communications Systems, Calculus, Advanced Calculus (it is actually signals and systems), Electromagnetics, ...etc.

This control one for example is a large book of over 500 pages and hundreds of solved and supplementary unsolved ones with answers and examples that has everything a heavy textbook that cost hundreds of dollars has.

Many of them can be found as downloadable PDF with a simple browser search