Lives of a Cell: Notes of a Biology Watcher by Lewis Thomas -- It's a little dated now, but this collection of essays is a great read. Actually, anything by Lewis Thomas is a great read.
Pandora's Breeches: Women, Science and Power in the Enlightenment by Patricia Fara -- a wonderful collection of biographies
The New Science of Strong Materials: Or Why You Don't Fall Through the Floor by J.E. Gordon -- I cannot recommend this book enough.
Prime Mover: A Natural History of Muscle by Steven Vogel -- Or, again, ANYTHING by Steven Vogel.
EDIT: I forgot one!
The Red Queen: Sex and the Evolution of Human Nature by Matt Ridley -- A bit controversial, this one. I disagree with some of what he says, but I quite enjoyed this book overall.