Barnes is also the author of an alternate history trilogy, collected as The Timeline Wars.
  1. Patton's Spaceship -- Mark Strang, an art historian turned professional bodyguard after half his family is killed by a shadowy terrorist group, learns that the terrorists are actually the arm of the Closers, a civilization turning timelines into totalitarian dictatorships. Closers are mysterious but thoroughly evil. Their civilization is based on Carthage. Their enemies, the ATN (Allied Timelines for Nondeterminism), are from an Athenian-dominated timeline. Strang is catapulted accidentally into the 60's in a world where Hitler won WWII, and has to essentially defeat the entire Axis army singlehandedly. The spaceship of the title is a recon craft (piloted by John Glenn) that is launched near the end of the book.
  2. Washington's Dirigible -- Strang, now working directly for the ATN, is sent to a Revolutionary-era America in a timeline where the ATN has boosted technology and prevented the war. Another Mark Strang, a Closer agent, is already there, and Strang is forced to fight himself to save the life of King George III, the best friend the colonies have. The dirigible of the title is the Great George, used near the end to spring the King from prison and reveal the ringer brought in by the Closers.
  3. Caesar's Bicycle -- The climax and finale. The ATN has lost touch with its futures. The cause seems to be a strong new ally, created by an ATN agent working in pre-imperial Rome. But Strang's mission this time, the early assassination of Julius Caesar, doesn't feel right to him. The bicycle is just one piece of technology introduced to the Romans, along with rubber, muskets, and stirrups.
They are nicely researched, and each one has different enough situations to keep you moving through the story. It also ties up neatly at the end -- unlike a lot of the multiverse dross out there these days, Barnes appears to have no plans of setting up a franchise.
Addendum -- I went back and reread the Timeline Wars this week. They are even better than I remember.