I really like a lot of the science/engineering books published by Dover Publications. They seem to specialize in republishing books originally published prior to 1980. The books are well-bound paperbacks, and are really low-cost. There are books on astrodynamics, space dynamics, mechanics, electromagnetism, chemistry, etc.
The few downsides are that the paperback covers themselves tend to be a tad flimsy. Also, as the contents are somewhat old, it is possible that they contain out-of-date information, though for many of the more fundamental subjects (esp. mathematics) this is not a problem. Finally, some of the books simply have higher-quality content than others.
I'm currently reading Principles of Electrodynamics
by Melvin Schwartz , which the author wrote in response to what he felt was a trend of textbooks writing about electricity and magnetism as if they were separate subjects (sadly, this is how I learned about electricity and magnetism). In this book, he uses special relativity to show how intimately the two subjects are tied together. I haven't gotten very far, but what I've read so far is quite interesting.
 Principles of Electrodynamics, by Melvin Schwartz