The Kill List by Frederick Forsyth

Kit Carson is a Marine working for a secret government agency named TOSA. His job is to kill those who intend to harm the U.S. and his code name is Tracker. His current target is a mysterious figure called Preacher, whose online "sermons" are inciting violence against innocent people in the U.S. and England. The problem is that no one knows exactly who Preacher really is--or where he's broadcasting from. Enlisting the help of various governments--as well as a brilliant teenage hacker with Asperger's--Tracker manages to gather clue after clue until he figures out the who and where. Then he just needs a plan.

It's a gripping story. And there's a little history lesson thrown in, so you learn something while you're being entertained.