Sticking with pop sci on evolution, and avoiding the evo-cre debate books, I'd like to add five books that are in the "exceptionally well-written and understandable" category (in no particular order):
E.O. Wilson, "The Diversity of Life", WW Norton 1999. Simply the best, most readable and comprehensive book on evolution for the non-scientist out there. So well written it might even make a believer out of a Believer.
Carl Zimmer, "At the Water's Edge", Touchstone Books 1998. Everything you ever wanted to know about how life came ashore and then decided it was a bad deal and returned to the sea
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Jonathan Weiner, "The Beak of the Finch", Vintage Books 1995. Covers the Grants' 30-year studies of natural selection in the Galapagos. If you had any lingering doubts about the creative power of natural selection, this should settle them. It's also a great example of science in action.
David Quammen, "The Song of the Dodo", Touchstone Books 1997. A brilliant introduction into island biogeography, the book covers the scientists, the science, and the studies that show islands are evolution's laboratories. It discusses what we know, what we don't know, and even corrects some of the things we thought we knew. Anyone interested in biodiversity, extinction, and biogeography needs to read this book if they read no others on the subject.
Connie Barlow, "The Ghosts of Evolution", Basic Books 2000. A fascinating, if sometimes whimsical, look at evolutionary anachronisms. The world abounds with leftover bits - plants and fruit whose fantastic adaptations were evolved to deal with long-vanished organisms. Highly recommended.