An Unexpected Light was the book that made me want to come to Afghanistan in the first place. I read it while I was still in secondary school and knew that this was a place I wanted to end up someday. A detailed account of Elliot's travels in Afghanistan during the 1990s, the book offers an indispensable introduction to cultural and historical principles in Afghanistan.
One of the first books I read when I first came to Afghanistan, this explores social environment and cultural identity, especially as it relates to the NGO and assistance community. It doesn't take you all the way up to the present day, but it's an excellent summary (read: indictment) of the post-2001 period.
None of the first three books go into much detail on the specifics of Afghan history, but Dorronsoro is the best primer on the past 30 years. Many books are written without the benefit of significant 'field' exposure to the Afghanistan that lies outside Kabul, embassies and MRAPs, but Dorronsoro (thankfully) does not fall into this category.
I'd be remiss if I didn't mention my own addition to the pile (full disclosure: I was one of the editors of this book). At a time when much of the world's attention falls on Afghanistan -- for good or for ill -- there are few books that convey a real and unfiltered sense of the Taliban movement and their roots in the villages of southern Afghanistan. This book does that. And the fact that I spent close to four years working to get this book translated, edited and published should tell you something about how important I think it is that policy-makers read this book.