I'm not saying these are the best five. Just the five I happened to read during the past few years, of the many that came highly recommended. I loved each one. In chronological order . . .
1. The Man Who Would Be King: The First American in Afghanistan. The story of a lone American who maneuvered his way into power in the early 19th century. Great story, of the five, the one I'd least recommend.
2. The Great Game: The Struggle for Empire in Central Asia. When you hear people talk about how Afghanistan is the graveyard of empires, this is one of the classic illustrations. It's a fascinating story of a different time—and a place people who really know Afghanistan will tell you to start if you want to understand it.
3. The Kite Runner. By now you've probably seen the movie and at least heard of the book. The movie was harder to watch. The book painted a more memorable picture of Afghanistan both pre- and post-Taliban.
4. Ghost Wars: The Secret History of the CIA, Afghanistan, and Bin Laden, from the Soviet Invasion to September 10, 2001. Absolutely gripping and well worth a read for its thorough research and wide-ranging scope, thought it's a bit tough to follow for the same reasons. It ends with the bad stuff that happened on September 10th that you probably never heard about.
5. Horse Soldiers: The Extraordinary Story of a Band of US Soldiers Who Rode to Victory in Afghanistan. Wow. If you want a feel-good story from Afghanistan, this is probably as close as you're going to get. Did your jaw drop reading accounts of the OBL-killing mission? This one's even better. What this handful of guys did is just astonishing.
Bonus: Haven't read it, but lots of people tell me The Looming Tower: Al-Qaeda and the Road to 9/11 is quite good. Covers more or less the same time period as Ghost Wars, but I think it focuses more on Al-Qaeda and less specifically on Afghanistan.