Try Mathematics for the Nonmathematician, by Morris Kline, especially chapters 3-5, although the entire book is terrific. It's a Dover book, and hence inexpensive.
A friend of mine has started studying for her GED and is looking for a book that covers pre-algebra and, if possible, algebra. It is important that the book takes the time to explain why things are done the way they are and how they work and not the plug and chug approach to mathematics that so many textbooks seem to take. Thanks.