The Valley-Westside War
Tor, Jul 2008, $24.95
There is no such thing as time travel into the past or future; but there is crosstime travel; alternate earths with differing histories. Crosstime traffic usually involves trading with other earths by pretending to be one of their citizens. One divergence led to a nuclear war in 1967 and the need to know why is critical.
The Mendoza brood gets a grant to study the causes of WW III sending civilization back into the Dark Age. A civil war between two local nations now has the Mendozas under suspicion. Liz Mendoza is doing research at the UCLA Campus Library when a soldier points her out to his superior as a possible spy. The Mendozas flee back to their home timeline but return to the nuked earth to gain some more data at a time when the war has just begun.
Harry Turtledove provides his latest exciting young adult multiverse science fiction thriller in which this time the divergence is a nuclear war. WWIII devastated the planet in 1967; the Crosstime Traffic agents wonder why this happened because the remaining records give no idea that hostilities will break out. Liz at times seems more of an adult than her parents as she does the critical tedious research to learn how a nuclear conflagration occurred in 1967.