David B. Mattingly
A nice hardcover omnibus of the first three books Keith Laumer wrote featuring Brion Bayard, crosstime agent: Imperium - containing Worlds of the Imperium, The Other Side of Time, and Assignment in Nowhere. I'll cheerfully recommend all of these as Laumer at his prime - in his serious/action mode.
Books 1 and 3 were paperback originals, which were later reprinted in hardcover only in Britain (which made them rather scarce.) By the time this American hardcover came out, I had already filled in my own collection with the expensive British editions, so someone else can enjoy this volume.
Worlds of the Imperium - Serialized in Fantastic February to April 1961. American diplomat Brion Bayard is kidnapped by an intelligence agency that works across the timelines. He is taken to an alternate Earth in which appeared identical doubles of famous personages - including a dangerous and hated dictator named Brion Bayard! His assignment seemed simple enough: dressed as his double, Brion was to enter the enemy stronghold, kill the dictator, and take his place until law and order could be maintained. But once having met his mirror-image, Brion had as little inclination to murder him as some other people had to let him live... For a first novel, this is rather impressive.
The Other Side of Time - expanded from magazine version in Fantastic April, May, June 1965. After a strange encounter, Brion Bayard finds himself in a deserted version of Stockholm-Zero-Zero, where he spends his time eluding another race with crosstime-traveling ability - the Hagroons, apelike slavers who plan on invading. He makes an escape in one of their machines, only to be captured and imprisoned with an agent of yet another crosstime race: monkeylike Dzok, of the Xonijeel. Together they escape and get to Dzok's homeline, only to run into more trouble, with Brion being "relocated." There he meets the lovely, lonely hypnotist Olivia, who lives in a strange, backward universe ruled by Napoleon the 5th. Brion ends up in a home-built crosstime machine, careening across timelines, struggling to find a way to prevent the invasion of his home timeline...
Assignment in Nowhere - an original novel with no previous magazine appearance. The viewpoint shifts to Johnny Curlon, a probablilty nexus. He's in serious trouble from some underworld heavies until Colonel Brion Bayard steps in and rescues him from a sinking boat... having followed those same heavies to find out what they were up to. It seems the heavies work for the Imperium, too. They were told to fetch Curlon, around whom great energies circled. His timeline was one a handful of viable ones surrounded by the Blight. The Blight has a way of nibbling at reality, and a probability storm was brewing. Baron General von Roosevelt saw in Curlon an opportunity to reshape the timelines to his own advantage. Only Bayard seemed to consider taking Curlon's side...