I've read 10 of Vince Flynn's "Mitch Rapp" series, and I love them. This prequel was one of the best. You would want to read the whole series in order, but it would be okay to start here. It is not quality literature, of course, and I have a few complaints about all of his books that I've noticed over time:

Vince Flynn always starts out with story lines and some character development that just end off into space. In the last quarter of all of his books, you can literally see Vince Flynn pulling all-nighters trying to wrap up the book before his publisher's deadline. He abruptly and inartfully crash-lands these books, abandoning some of the plot development more carefully written earlier in the book. In his previous couple of books, "Extreme Measures" and "Pursuit of Honor," which were really one book, he spends a ton of time making repetitive and boring arguments in favor of aggressive interrogation and covert action in the war on terror. (I'm in favor of doing all that, for the record.) And at that, it wasn't particularly well, and didn't contribute anything very creative to the debate. "American Assassin" drops the boring dialogue and gets back to what we love about Mitch Rapp: daring operations against bad guys. Love it. As far as explaining his motivation for going after the terrorists, he devotes about half a page to saying he loved his deceased girlfriend, and that's it. She's already gone when the novel starts, and you get a couple of flashbacks that don't really lay a good groundwork for his relationship with his girlfriend that got killed by terrorists. Nor do you get a well-developed picture of how Irene Kennedy found and courted him. It just pretty much starts with the training, and then there's a missing reel or two, and then they are conducting operations against the bad guys. Oh well...still loved it.