As a book junkie, I love to get and give book recommendations. Here is my Darwin Day recommended books list:
What Evolution Is, Ernst Mayr - My favorite brief, single-volume primer on evolution for non-specialists. Mayr covers the big topics - selection, adaptation, variation, speciation, fossils, evolutionary trees, evo-devo, and human evolution. Mayr was a major figure in mid-20th century evolutionary biology. He was in his 90's when he wrote this book, nevertheless, this book is remarkably clear and up to date.
The Ancestor's Tale, Richard Dawkins - In a nod to the Canterbury Tales, Dawkins takes us on a backwards pilgrimage from today to the dawn of life on earth. Along the way we meet other 'pilgrims' of today's species, as we join up at points of common ancestry. Dawkins came up with a brilliant way to tell the story of our evolutionary lineage all the way back.
Summer for The Gods, Edward Larson - This Pulitzer Prize-winning book is one of the best histories of the Scopes trial. Larson also wrote Trial And Error, the most comprehensive survey of evolution, creationism, and the law in the United States.
Endless Forms Most Beautiful, Sean Carroll - one of the best books covering evolution from a molecular and developmental perspective, by one of the top researchers in the field. It is a nice introduction to the genes that make our body plan, and how these genes are versatile enough to generate the diversity of body plans we see among animals.
Human Natures, Paul Ehrlich - This book is very much Ehrlich's personal perspective, but it delves deeply into some older research on human evolution, and the evolution of human intelligence in particular. Ehrlich knows how to grab your attention and keep it.
Darwinism and the Divine in America, Jon Roberts - This historical monograph covers the intellectual history of evolution among religious leaders in America from 1859 to the end of the 19th century. The book is dense, since it written for Roberts' peers in professional history. It tells the fascinating story of how theologians at first gave evolution short shrift, thinking that it was only a temporary scientific fad. When they realized that scientists had accepted the evidence for evolution and that the theory had staying power, they began digging deep to deal with the implications that this new science held for their theology.
Tower of Babel, Robert Pennock - This is the definitive philosophical rebuttal of the claims of intelligent design. Pennock is a philosopher, and engages in-depth the arguments made by creationists in the 1980's and 1990's.
Before the Dawn, Nicolas Wade - Wade, one of the top science journalists for The New York Times, has written a great book on what the latest in paleontology, anthropology, and genetics tells us about recent human evolution.
River Out of Eden, Richard Dawkins - This is a short book that summarizes many of Dawkins' longer and more technical arguments from his book, The Selfish Gene. River Out of Eden contains one of the most compelling chapters I've ever read on evolution, called "God's Utility Function." People may disagree with the argument, but if you really think earth was specifically desgined for humans, you have to deal with Dawkins' argument.
The Creationists, Ronald Numbers - The definitive history of the movement, by an empathetic (not sympathetic) historian. Numbers does a great job detailing the 20th century history of creationism in America, and he deals very well with the internal conflicts of the leaders of the various creationist and theistic evolution movements, many of whom Numbers interviewed.
Evolution vs. Creationism, Eugenie Scott - Desgined as a textbook on the subject, Scott lays out the basic issues without getting polemical. The book includes readings from scientists and creationists. This is a great intro to the basics of the cultural (not scientific!) debate.
The Metaphysical Club, Louis Menand - OK, this is an odd pick, but this Pulitzer Prize-winning book deals in part with the influence of Darwinism on an important group of turn-of-the-century American thinkers such as John Dewey, William James, and Oliver Wendell Holmes.
An finally, if you're really serious about the technical aspects of evolution, I'd recommend Principles of Population Genetics, by Daniel Hartl and Andrew Clark. It doesn't deal with fossils or 'macroscopic' biology; this is book is covers quantitative theories about genes and evolution. If you want to learn the real math behind evolutionary theory, this is a great place to start.
There are a ton of other books on my shelf I've left out, and many other good ones I haven't read yet. Feel free to post you're favorites in the comments.