Michael Jackson: Galapagos, a Natural History Guide
A one-volume essential reference to the islands' natural history. Lots of illustrations.
Pierre Constant:
The Galapagos Islands - A Natural History Guide
Another excellent reference book. Covers geology, oceanography, history and flora and fauna. Again, lots of illustrations and color photos.
Charles Darwin:
Voyage of the Beagle
First published in 1839. This is Darwin's account of the voyage that changed our way of looking at the world.
Tui De Roy: Galapagos, Islands Born of Fire
Oversized hardcover featuring incredibly moving photo essays on the wildlife and habitats of the islands.
David Steadman: Galapagos, Discovery on Darwin's Islands
Hundreds of photos and full-color paintings plus insightful discussion on natural history and evolution.
Jonathan Wiener: The Beak of the Finch
Pulitzer Prize-winning work using the 20+year study of Peter and Rosemary Grant of the finches on Daphne Major as a springboard for the discussion of the mode and pace of evolution. (hint: evolution is happening RIGHT NOW). Very accessible. Great science writing.
Kurt Vonnegut: Galapagos, A Novel
Biting satire set on the islands and in Guayaquil. A million year "flashback" to the nature cruise of the century and the near extinction of the human race.
Cathleen Schine: The Evolution of Jane
Part travelogue, part psychological insight, part evolutionary commentary. Centers around a recently-divorced young woman and her discoveries and self-discoveries on the islands.
Edward J. Larson: Evolution's Workshop
"God and Science on the Galapagos Islands" A compelling history of the islands' last three centuries. It presents the various thoughts on evolution through time ranging from pre-evolutionary thinking, to creationism to enviromentalism and everything in between.
These are not specific to the islands, but are interesting books on evolutionary theory:
David Quammen: The Song of the Dodo
Subtitled "Island Biogeography in an age of Extinction." Lots of personal observations, scientific theory and discussion of the intricacies of extinction and evolution as they are illuminated by the study of islands. This is a great book - one of my all-time favorite non-fiction reads. Mr. Quammen is an excellent writer and this book reads more like a novel than the wake-up call that it is.
E.O. Wilson: The Diversity of Life
A classic about the beauty and importance of biodiversity.
Other authors who you might find interesting are Ernst Mayer and Stephen Jay Gould (plus anything else by David Quammen or E.O. Wilson).