(Iron is the only element that cannot cross between Earths. Pratchett and Baxter never explain how human hemoglobin isn't destroyed the first time a human "steps"...)
Things also got weirder as the series went along. Previous volumes have introduced a wide cast of characters, most of whom appear in _Cosmos_. _The Long Earth_ introduced the original "stepping box," powered by a potato. In _The Long War_, trade by airships capable of stepping becomes commonplace, and human colonies begin to declare their independence from the Datum. _The Long Mars_ introduced a second string of worlds, independent of the Long Earth; and also introduced the Next, our successors. And _The Long Utopia_ is mostly about the relationships between the Next and the "Dimbulbs," plus bonus! dangerous alien machines.
_Cosmos_ begins with the nearly-universal receipt of a message from space: JOIN US. The superintelligent Next begin working on decoding the technical content of the message, while humans, trolls, and beagles (a race of humanoid, intelligent dogs from millions of Earths out) are caught up in the debate over whether it's even good to answer the message, particularly in light of what happened in _Utopia_.
Meanwhile, the series' main characters, Joshua and Nelson, both find themselves unexpectedly grandfathers - Nelson in a mysterious communication from the AI Lobsang, and Joshua when his estranged son Rod comes to rescue him from a distant Earth where he has been gravely wounded. Before all is wrapped up, something comes to be known of the nature and purpose of the Long Earth, and humans travel to the stars.
It's all good fun, and if it isn't as full of ha-ha as Pratchett's solo books (or his collaboration with Neil Gaiman, _Good Omens_...), it's lighter than most of Baxter's solo work. I'm delighted to say that the series maintains its quality throughout, and ends on a high note.