Harrison's. I don't consider it a textbook but more of a casual or light summer read.
On a serious note, here are some fun options:
The Body: A Guide for Occupants by Bill Bryson. Bryson is a humorist and writer. It's very light, fun, and witty.
Clinical Pathophysiology Made Ridiculously Simple by Aaron Berkowitz. If, for some inexplicable reason, you actually want to learn some medicine before med school, this book is at least light enough of a read that it shouldn't be too mentally taxing. Berkowitz is a neurologist at B&W's, one of the Harvard hospitals.
The Deadly Dinner Party: And Other Medical Detective Stories by Jonathan Edlow. Edlow is an internist as well as an emergency physician at BIDM, one of the Harvard hospitals. The book is in the vein of Sherlock Holmes (and Sir Arthur Conan Doyle was a physician and based Sherlock Holmes partly on one of his med school professors, Joseph Bell).
Extreme Medicine: How Exploration Transformed Medicine in the Twentieth Century by Kevin Fong. Fong is an anesthesiologist and critical care physician based in the UK. He's an expert in space medicine.
The Ghost Map: The Story of London's Most Terrifying Epidemic - And How It Changed Science, Cities, and the Modern World by Steven Johnson. All about John Snow and the cholera outbreak in Soho, London. And epidemiology. Johnson is a journalist and popular science writer.
The Mystery of the Exploding Teeth: And Other Curiosities in the History of Medicine by Thomas Morris. Morris is a journalist and medical historian.
Operations That Made History by Harold Ellis. Title says it all. Ellis is a retired British surgeon.
But more fun, beach readin' stuff.
