Hyper versum, by Cecilia Randall (2006, Florence)

Daniel's an american university student. He loves playing Hyperversum, a videogame that can virtually move players to every historical and geographical location they choose. But one day, while he, his brother Martin, his girlfriend Jodie, his best friend Ian and a couple of friends are playing, sth that can't be explained happened and all of them are really moved into France of 1214, with no chance to get back.
Very particular, detailed, changing points of view and page-turner.