The Author's Note of Prelude to Foundation contains Asimov's suggested reading order for his science fiction books. [source: Wikipedia]

1. I, Robot (1950)
   
2. The Positronic Man (1992)
   
3. Nemesis (1989)
   
4. The Caves of Steel (1954)
   
5. The Naked Sun (1957)
   
6. The Robots of Dawn (1983)
   
7. Robots and Empire (1985)
   
8. The Currents of Space (1952)
   
9. The Stars, Like Dust (1951)
   
10. Pebble in the Sky (1950)
    
11. Prelude to Foundation (1988)
    
12. Forward the Foundation (1993)
    
13. Foundation (1951)
    
14. Foundation and Empire (1952)
    
15. Second Foundation (1953)
    
16. Foundation's Edge (1982)
    
17. Foundation and Earth (1986)
    
18. The End of Eternity (1955)