One day a little potato-powered machine schematic appears on the internet; children all over the world build it out of spare electronics and--POP--begin disappearing. The machine takes people into other parallel-universe Earths, much like the advanced-Neanderthal world described in Robert Sawyer's "Neanderthal Parallax" trilogy. In Pratchett/Baxter's take on the idea, these other Earths form an infinite, unpeopled chain. A new frontier.
The authors use this endless string of Earths to theorize, to play with ideas. And they cast a wide net. What would happen if all of mankind's space and resource constraints instantly disappeared? Demographically? Economically? Legally? Then they start visiting these worlds, which lets them delve into geological history, evolutionary history, you name it. Though it was all very lightly done--they only had 336 pages to play with--I thought it made for utterly fascinating digging. Also, there's a cannon-bird.
There is a plot of sorts. The conflict does not receive much of a resolution, but that's okay because it wasn't much of a conflict to begin with. It was just another excuse for the characters to keep exploring, and another chance to hypothesize a different form of life.
It ends with a hell of a cynical comment on the nature of humanity; in spite of this it does not feel like a cynical book.