The Traveler by John Twelve Hawks (2005) Doubleday 

There are people who want to control the world by forcing it to follow orders.  There are Travelers who bring change to the world.  The Tabula see Travelers as dangerous to world order.  And there are Harlequins who protect Travelers.  This is a story of a Harlequin who is called to serve and protect a set of brothers who may be Travelers against the Tabula agents sent to kill them.