1. The Essential Calvin and Hobbes (1985)
2. Calvin and Hobbes (1986)
3. Something Under the Bed Is Drooling (1987)
4. The Days Are Just Packed (1988)
5. The Year of the Barely Controlled Chaos (1989)
6. The Revenge of the Baby-Sitters (1990)
7. Attack of the Deranged Mutant Killer Monster Snow Goons (1991)
8. There's Treasure Everywhere (1992)
9. The Calvin and Hobbes Tenth Anniversary Book (1993)
10. The Complete Calvin and Hobbes (1995)
There are also several additional books that collect together various comic strips or essays by Bill Watterson:
* The Calvin and Hobbes Lazy Sunday Book
* The Calvin and Hobbes Sunday Pages
* The Calvin and Hobbes Collection
* Calvin and Hobbes: The Complete Collection (a single volume collecting all the strips)
* The Calvin and Hobbes Newspaper Strips
It's important to note that the first five books are collected into "The Complete Calvin and Hobbes", so you don't necessarily need to buy all of them individually.