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. Since their world was running low on resources, they peacefully sought supplies from other Earths and saved their civilization from collapse. Harry Norman Turtledove (born June 14, 1949) is an American historian and prolific novelist who has written historical fiction, fantasy, and science fiction works. ...
The current books in the series are:
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 Franciso in a world where Imperial Germany reigns supreme following its victory in World War One.
In High Places (2006): Takes place in a world where the Black Death killed four fifth's 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.
Just as importantly, they must guard the secret of Crosstime Traffic--for of the millions of parallel timelines, this is one of the few advanced enough to use that secret against us.Now, however, the German occupation police are harrassing them.
Now the CrosstimeTraffic corporation watches the Germans carefully, and does everything it can to be sure the Germans don't learn the secret of the ability to pass across alternate planes of earth.
While it watches, Crosstime trades with this plane, dumping archaic VCRs and other electronic equipment that is completely outdated in the home plane, but fully up-to-date in a world where science has moved a little more slowly (in the absense of cold-war competition).