A fantastic read. Much more techno-thriller than science fiction, although the science fiction is fairly critical to the plot ('cowboy angels' are black ops agents trained to infiltrate alternate worlds). The story ends up being very human - the hero is a retired agent, reactivated to try and bring an old friend back in. The one thing that seemed to run through it was his determination to do the right thing - not just his duty, but the right thing for the people around him too.
The friend has been murdering alternate copies of the same woman in different worlds. As we get further and further into the plots and counterplots, things stay exciting. That keeps up all the way to the last paragraph.
The way that the alternate worlds fit together, and how the one that invented the technology to travel to others deals with it is fascinating too. The technology was invented in the USA and they used it to help out alternate, and less fortunate, Americas. A lot of that help is trade, of course, so they enriched themselves too. He shows several different facets of these interactions (including with our own world, in which they are acting covertly).