Island in the Sea of Time by S.M. Stirling starts with a mysterious, never explained event which takes the island of Nantucket from 1997* and moves it, and its inhabitants (including a Coast Guard training ship just off shore), back in time to into 1250 BC.
It’s a really good book but I don’t have much to say about it at this time. I liked that the main cast of characters were diverse. I like that the more technologically advanced Nantucketers didn’t assume that their superior technology made them intellectually superior to the natives of the time period. I liked the reversion of some technologies (Nantucket could not produce guns at the same level as the 20th century and had to revert to Civil War era technology) and how the mistakes of the past were used to recreate the wheel, as it were. I thought the use of rape and torture as plot devices came up a too much. I didn’t like the sequels as much as they felt like a series of battle scenes strung together. But I did really like this one and if you enjoy speculative fiction, I encourage you to give it a try.
*Or so, I don’t remember exactly when the opening of the book took was set. I’m guessing based off the fact that the book was published in 1998.