I picked up this one at the library after I read and liked something else by this author (The Quiet War) late last year.
The basic premise behind Cowboy Angels is around the existence of what are called Turing Gates, essentially devices that allow people and things to travel into alternate universes. Each universe, usually described as a 'sheaf', has differentiated itself from the one we know in some way - the main place we see, calling itself the Real, is very clearly not the US we're familiar with, though that particular sheaf also plays a significant role in the storyline.
The main characters include Adam Stone, one of the eponymous Cowboy Angels, a group of orphans trained by the CIA-equivalent in the Real to be special operatives in a number of alternate worlds. For years, the policy of the Real had been to try and create a Pan-American alliance between universes, but then the election of Jimmy Carter puts a stop to all that and Stone heads off somewhere quiet till he's called back into action when a former colleague is working his way through sheaves, killing specific individuals in each.
Unfortunately, while I found the novel itself quite gripping, particularly in its depiction of the machinations of the various agencies, the ending kind of happened and didn't really resolve all that much. It hasn't stopped me from wanting to read more by this author - I'm planning to start with his first book next, 400 Billion Stars, the first in a trilogy.