This book was the first book of Powers' that I read, recommended by a friend. I thought it was really good and so went on to read a lot of his other stuff. Unfortunately, he's not always stayed as good.
The story involves Brian Duffy, an Irish soldier of fortune, who is lured to Vienna to act as a bouncer by an mysterious old man. Vienna is under siege by the Turks at the time and eventually Duffy discovers the real reason for this (which I won't reveal) and his role in preventing them accomplishing their true purpose. This mediaeval mystical conspiracy theory sounds vaguely ludicrous, as Duffy himself relates to a friend, and this level of wry humour is what has been missing from Powers' recent books. Powers has continued with his mystical conspiracy theories of history, incidentally, "The Anubis Gates" and "On Stranger Tides" being good examples of this (and good books.) He's less successful importing this mystical conspiracy theory stuff into the modern day.
Back to this book. It's well written, interesting, historically accurate in the historical parts (and who can say about the mystical conspiracy theory) and wryly humourous in place. In short, all you could want from a book. Buy it.