While I may not agree with some of his suggestions on foreign policy (for instance, his most recent article positing that the Middle East requires Western imperialism in order to possess order, which smells an awful lot like an articulate argument that those poor savages need the iron hand of the refined, developed world in order to peacefully exist) Robert Kaplan is a journalist whose "been-there-done-that" street cred is undeniable.
As a foreign correspondent, he's written for The Atlantic, The Washington Post, The New York Times, The New Republic, The National Interest, Foreign Affairs, Reader's Digest, and The Wall Street Journal, was a Senior Fellow at the Center for a New American Security in Washington, and was appointed to the Defense Policy Board—a federal advisory committee to the United States Department of Defense. Basically, the guy knows his stuff, and has a wealth of experience to back it up.
"Soldiers of God" is one of his earlier works, (written in 1990) following his time in Afghanistan during the Soviet invasion. His writing style in this book is plain, highly analytical, and frankly, dry—heavy with information and filled with difficult-to-remember names—but if you can muscle your way through, you may find as I did his explanation of the Soviet invasion of Afghanistan, the parties involved, the nature and goals of the Mujahideen, and the country's brutal climate are enlightening and extremely detailed.
His portrayal of the Afghan jihadists has drawn criticism from some readers (who likely don't understand the notion that this book was written ten years before the 9/11 attacks on the World Trade Center, back when the Mujahideen were receiving CIA assistance and weapons to fight our common enemy), but I actually found his approach well-rounded. He hardly heaps praise on the men he followed and interviewed, and pulls no punches in criticizing certain aspects of their culture and their behavior.
I purchased and read this book to be used as research for my own books, and in that regard, it didn't disappoint.
You can find "Soldiers of God" here.