"Beyond The Gap" is slightly different. It depicts human society, yes, but not on any earth we would recognise. The communities in the novel range from pre-historic (the Bizogots herd mammoths) to medieval (Raumsdalians live in castles and fight with swords). All of them share an existence that is dominated by a huge earth encircling Glacier (always with a capital G) to the north. Except that the Glacier has been retreating for the last few centuries and now a gap has appeared that allows travel to the other side. The descriptions of living and travelling in the shadow of the Glacier are very well done, you almost feel the chill as you read it.
"Beyond The Gap" is also the first book in the "Opening Of The Earth" trilogy and, given that the three books tell a single story, should really be read with "The Breath Of God" and "The Golden Shrine". There is a definite stylistic difference between the first two books and the last one; the first two are written in the style of Turtledove's "Crosstime Traffic" series which is aimed at a younger audience but "The Golden Shrine", while maintaining some aspects of this style such as the annoyingly frequent repetition of key phrases like "he couldn't tell him he was wrong", has a much more adult feel about it. The critical plot element that resolves the story was obvious, at least to me, by about half way through the first book but was well done nonetheless and was immediately followed by a final plot twist of biblical proportions that I hadn't anticipated and greatly added to my enjoyment of the story and my appreciation of Turtledove's skills as a writer.
This series was an enjoyable read over a week or so on the train. I think I've had enough Turtledove at the moment, though, so although I've got "Give Me Back My Legions!" sitting beside the bed, I suspect I'll return it to the library unread. Turtledove fans can, however, expect me to return to the author later in the year.