Crosstime Traffic is a series of books by Harry Turtledove. The central premise of the stories is an Earth that has discovered access to alternate universes where history went differently. "Crosstime Traffic" is the name of the company with a global monopoly on the technology.
The background strongly resembles that of H. Beam Piper's Paratime series and Keith Laumer's Imperium Series. One tribute paid to Piper's series is the names of the inventors of temporal transposition: Ghaldron and Hesthor in Piper, Galbraith and Hester in Turtledove. In all of the series the "home timeline" was running low on resources, and it has used its knowledge of time to covertly import supplies from other Earths and save their civilization from collapse. The most important difference is the nature of the home timeline. Piper's world was inhabited by a culture which had been technologically advanced for thousands of years and was even more distantly related to our own. Laumer's series had a civilization that was less advanced than our own in almost every way except for their travel technology. Turtledove's world, although set in the 2090s, resembles our 2010s, with modest general advances in technology plus the crosstime capability.
The books are young adult novels with teenage protagonists, who frequently become stranded in dangerous alternate worlds and must adapt to survive. Their adventures give them increased appreciation for the benefits of living in a civilized, high-tech society. Invariably, each book has two viewpoint characters, a boy and a girl - different ones in each book; in most books one of them is from the home timeline and the other from a visited alternate, which greatly decreases the chances of love interest developing between them being successfully consummated.
While there is considerable violence, the language and plots are restricted by the intended audience. For instance, In High Places includes the prospect of an enslaved girl being sexually abused, but does not use the word "rape" (although the word is later used in The Valley-Westside War).
Novels in the Crosstime Traffic series
- Gunpowder Empire (2003): The first book in the series, it involves a pair of siblings stranded during a siege of an outpost of a Roman Empire that never fell.
- Curious Notions (2004): The second book in the series is about a teenager and his father who are running an electronics store in San Francisco in a world where Imperial Germany reigns supreme following its victory in World War One.
- In High Places (2005): Takes place in a world where the Black Death killed four-fifths of Europe's population, and the Moors still occupy Spain and southern France, and the Industrial Revolution never happened.
- The Disunited States of America (2006): This book concerns a pair of teenagers, one from the Cross-time civilization, one a native, who meet in a Virginia where the United States fell apart, in a North America torn by war between numerous independent states. The working title for this book was The Untied States of America.
- The Gladiator (2007): This novel is set in a world dominated by the Soviet Union, after it won the Cold War in the late 20th century. In Italy, two teenagers chafe under the deadening rule of communism— until they discover the existence of Crosstime Traffic through a strategy gaming shop which is not as it seems.
- The Valley-Westside War (2008): The sixth book in the series, set in Los Angeles a world in which a nuclear war took place in 1967. LA and the rest of the USA are split into several tiny republics, kingdoms and such, we are told a story of when the Valley invaded the Westside.
- Crosstime Traffic page on Official website.