So The Long Mars is the third - and probably final - volume in Terry Pratchett and Stephen Baxter's Long Earth series. And, like the first two books - The Long Earth and The Long War - it centres around a pioneering voyage: a US Navy crew attempting to travel further into the Long Earth (a series of parallel Earths, each one another evolutionary possibility which did not happen here, which humans can 'step' between using a nifty bit of circuitry and a potato) than Anyone Has Done Before. Again.
There's also the story of a trip to the Long Mars, which is basically the same as the Long Earth except with Mars, conducted by pioneer Sally Linsay and her father Willis. And, just to increase the concentration of far-out-ness, a new race of post-humans, super-intelligent and manipulative, may be rising out of the shadows of the far Long Earth.
I've always thought that the best thing about the Long Earth trilogy is not its plot, nor its characters, but its worldbuilding. I don't believe anyone is really interested in the lives of Joshua Valiente, natural stepper, or Sally Linsay, or even Monica Jansson, cop. Perhaps Lobsang, Tibetan motorcycle repairman reincarnated as supercomputer, might grab your attention, but only for a moment. And as for the plot - well, that's mostly non-existent. These are tales of journeys into the unknown, but nothing really happens except for a bit of sightseeing. But the worldbuilding - that is brilliant. The Long Earth series is clearly the result of two geeky guys sitting down and painstakingly, properly working out the exact and miniscule consequences of having sudden, widespread access to an indefinite and probably infinite number of universes. Political, economic, social - as a series, the books cover them all convincingly and fascinatingly. (Is that a word? It is now.)
It's a shame, then, that The Long Mars is comparatively thin on such social analysis. Gone are the vignettes of ordinary people living out pioneer lives, coping with problems new and ancient as they move out into the unknown. Instead, we concentrate solely on the voyages, which, as noted before, aren't really that interesting. Sure, it's quite nice to read about possible life on Mars, and how it might work, but after the millionth iteration of the same scene from Dune it gets a little tiresome. (Speaking of borrowings: I'm disappointed to note that the crab civilisation the Cernan finds is lifted straight from The Science of Discworld. Poor form, Sir Terry. An infinite multiverse, and you can't think of another species to populate the Earth?) And, while the dialogue of The Long Mars is a little less dire than it was in the rest of the series, it's too little too late to make such two-dimensional characters at all relatable.
I'm giving The Long Mars 3.5 stars for the sake of the rest of the trilogy, and for some interesting science-y ideas. The Waterstones Special Edition Epilogue is good, too, if a bit of a shock. But this is a series that seems to go out not with a bang but a whimper. And an obvious whimper at that.