What Ansel Adams book if I only buy one?

I think "The Negative" would be one of his most important ones. He goes into great detail in explaining his system of exposure: "The Zone System". It an historically important book as well as being very educational.

The Ansel Adams Guide : Basic Techniques of Photography - Book 1 (Ansel Adams's Guide to the Basic Techniques of Photography) by John P. Schaefer

http://www.amazon.com/exec/obidos/tg/detail/-/0821225758

Ditto: The Negative

_The Negative_.

The effect of different lenses{1} are covered in 'The Camera'. Exposure goes into 'The Negative', and printmaking goes into 'The Print'

I'd consider the three volumes as a single unit and buy all three. If you do not have a darkroom or develop you own film, the usefulness of that volume is limited to occasional glimpses on composition, previsualization, placing exposure values in the negative, and light and filters considerations.

{1} Not many, AA did most of his work in large and medium format, and the lens selection is limited compared to 35mm. On the other hand, there are chapters dedicated to view camera movements.

And again: the negative.
It does not teach at all about the effects of different lenses though. But if only one, the one about exposure absolutely is the most important.

Adams clearly meant for students to read & study the "textbooks" in this order: "The Camera", followed by "The Negative" followed by "The Print". The body of knowledge is hierarchal but you need a solid footing. So my suggestion is to follow his recommendation and start with "the Camera".

As a companion book to all three my suggestion is "Examples : The Making of 40 Photographs". where he discusses the real world application of the techniques discussed in the series to his approach to photography.

The "zone system" is simply a way to make sense of the technical aspects of photography the same way a ruler tells you how long, wide or tall an object is, and whether the ruler is marked in centimeters or inches the size of the object doesn't change. In photography, according to Adams, the object of photography is to see and express your "vision of what you see" clearly. Creating your own way of " Photographic seeing" is about learning and understanding how to think about a photograph, the way a camera, film and a print will shape your interpretation of what is in front of you. AA developed the Zone System as an aid to teaching the mechanics of photography . Mastering The "Zone System" is not an end in itself , although some people mistakenly think it is. You still need to have something to express with your photograph, and not merely exercise technique.

To this end a last book I'll recommend is "The Nature of Photographs" by Stephen Shore.

THE NEGATIVE which is Book 2 in the series, is the definitive text on Exposure and The Zone System. Without good comprehension of THE NEGATIVE all other books on photography are--essentially--useless.

You can't just "read" the book, you have to UNDERSTAND it and how everything is interrelated to capture the important parts of a scene's brightness range on film--black and white OR color. While THE NEGATIVE describes b&w in detail, the principles of understanding brightness range and "placing" the most important parts of the subject area in the correct zone (by calculating the exposure from that point) are all the same.

Snapshots and "pictures" are "taken" by people who own cameras. "Photographs" are "made" by artists who are skilled technicians and who understand and can apply the principles laid out in Ansel Adams' book THE NEGATIVE.

"The Negative" - if you want to learn more about technique. "Yosemite and the Range of Light" if you want to see beautiful photographs. "Examples - The Making of 40 Photographs" is also good if you can't find Yosemite and the Range of Light.