Imperium, Baen, 2005 was a collection of three short novels by Keith Laumer first published in the 1960s. The collection was edited by Eric Flint with an preface by Harry Turtledove titled "This is Where I Came In". The premise behind Laumer's stories is that there is a multiverse where different strands of history co-exist based on historic events going in both (or more) directions at once. Turtledove later used the concept in his Crosstime Traffic Series.
In the preface, Turtledove explains how he read the first two stories when they came out in the 1960s and how they, along with Lest Darkness Fall by L. Sprague de Camp and The Man in the High Castle by Philip K. Dick, introduced him to the alternate history sub-genre. He thinks that H. Beam Piper first showed a vehicle that traveled between timelines in his Paratime stories but felt that Laumer's were the Ford Model T of the genre.
Turtledove describes how, in a timeline not our own, two scientists named Maxoni and Cocini (similar to his Galbraith and Hester) discovered a way to travel between the timelines in the 19th century. This discovery became the property of the British, German and Swedish royal houses which united into the Imperium. However, the process was very dangerous and if improperly implemented would release enormous amounts of energy, enough to destroy the Earth. This meant that the timelines nearest to the World of the Imperium were devastated and are called "The Blight" by cross-time travelers. The protagonist of the first two stories is Brion Bayard, an American kidnapped from our timeline, which is one of two others within The Blight that were not devastated, to assist agents of the Imperium deal with a dangerous series of attacks from the third "Blight-Insular" timeline.
Turtledove then gives a short synopsis of each of the three novels and concludes that while at a total of 140,000 words, they would make a medium-sized book by today's standard, they contain enough interesting ideas for six to eight doorstoppers. He also emphasizes Laumer's well developed characters and his dry wit and encourages the reader to enjoy the book as a good introduction to Laumer's work.