Best book by Dawkins is either The Blind Watchmaker or the Selfish Gene.
The Blind Watchmaker is very informative, esp. for those of us not majoring in the biological sciences. It explains everything you'd get probably from a biology 101 textbook in a very clear fashion, and it refutes intelligent design, i.e. creationism, at the same time.
It also has a program that called "biomorphs" that you can order. The book describes how to use it, which "demonstrates very effectively how random mutation followed by non-random selection can lead to interesting, complex forms" according to one website.
Dawkins himself said of the program: "I was almost feverish with excitement. I cannot convey the exaltation I felt of exploring a land which I had supposedly made. Nothing in my biologist's background, nothing in my 20 years of programming computers, and nothing in my wildest dreams, prepared me for what actually emerged on the screen."
Which book of Richard Dawkins I would recommend depends on what you want to read. The rest of his books are mostly about biology. I would suggest "A Devil's Chaplain", because it is an easy read with a lot of variety in content, which 30 or so essays or "Climbing Mount Improbable".
If you want an introductory book on evolution, I would recommend "Evolution: The Triumph of an Idea" by Carl Zimmer or "Evolution: A Scientific American Reader" by Scientific American. If you are familiar with evolution as such, I would recommend "Evolution: What The Fossils Say and Why It Matters" by Donald Prothero or "Darwin's Dangerous Idea" by Daniel Dennett. If you have the cash and would like to study the topic of evolution extensively, I would suggest "Evolution" by Douglas J. Futuyma or "Evolution" by Nicholas H. Barton et. al.
If you are looking for books that are critical of religion and theism like "The God Delusion", I would recommend books such as "The End of Faith" by Sam Harris, "God is not Great: How Religion Poisons Everything" by Christopher Hitchens or "God - The Failed Hypothesis: How Science Shows that God Doesn't Exist" by Victor Stenger.
For books critical of the intelligent design creationists and their 'arguments', I would suggest "Why Darwin Matters: The Case Against Intelligent Design" by Michael Shermer for an introduction or "Why Intelligent Design Fails: A Scientific Critique of New Creationism" by Matt Young and Taner Edis for the more advanced reader. A look at the history of intelligent design creationism, read "Creationism's Trojan Horse: The Wedge of Intelligent Design" by Barbara Forrest and Paul Gross.
It all depends on what your level of knowledge is, and your specific field(s) of interest. If you want me to be more specific, you would have to be more specific in what sort of book(s) you are looking for.
"The selfish gene" is the most important book - if you get this you understand evolution.
Blind watchmaker he seemed to be preaching to the converted, I did enjoy "The Ancestors Tale", not as ground breaking but a fascinating tour of evolution.