How To Hydroponics is likely the best book I've ever read on this subject. Not because it's so full of information--which it is, but because of it's down-to-earth and very readable nature.
Other hydroponics books cover much more information with lots of scientific graphs and information. This one, though, focuses on the actual hydroponics gardener rather than the data-hound scientist type.
The science of hydroponics can be interesting, sure, but for those of us who actually want to use the information in a practical sense, most of that science is extraneous.
How To Hydroponics includes general information so you'll know how hydroponics works, why it works, and what you need to know to make it work. That's the first third of the book.
The second third is about the general uses of hydroponics, ideas for how it is used, where it is used, and so forth. The different types of hydroponics systems and what situations they're best used for are covered too.
The final third is the meat of the matter and the culmination of the book's information. This is where Keith Roberto (the book's author) gets down to the nitty-gritty. He shows you how to build a variety of hydroponics systems to fit your situation. Ebb-and-flow, aeroponics, constant immersion, and more.
Totally awesome book and very highly recommended. Great stuff!