This is a wonderful book for getting you to understand why functional analysis is extremely useful for numerical calculations. It is also just well written, with good exercises that will teach you how to think about solving these types of problems. Sawyer does an excellent job of introducing and explaining concepts, esp. by using analogies with lower dimensions to the harder-to-visualize large or infinite dimension cases.
If you have an interest in functional analysis, this book is a great introduction. I especially like the Newton-Raphson explanation for finding functions that solve differential (or integral) equations.