The third instalment in what is rapidly becoming my favourite book series was as much as I'd hoped for. I'll admit to being something of a Pratchett and Baxter fanboy (independently), and while that didn't necessarily mean it would be a given that I'd like their collaborative efforts, it's fair to say I'm not entirely unbiased. 
 The book picks up immediately after the ending of The Long War, with the devastation of the Yellowstone eruption on Datum, though it quickly skips on some five years, occasionally jumping back to fill in the gaps. 
 There are three plots in play, telling the tale of most of the familiar characters from the previous two books. One thread follows Joshua as he investigates the Next, a group of people that claim to be a new, more intelligent species of human. Another thread joins Maggie Kauffman on a new state of the art ship, and her journey deep into the Long Earth that dwarfs even the Chinese achievements in reaching 20,000,000 in the previous book. Finally, the thread that the books takes it title from, follows Sally Linsay, reunited with her father for a trip across the Long Mars. 
 To be entirely honest, I wouldn't recommend this book to someone who hasn't read (and liked) the previous two books. Pratchett and Baxter are brilliant writers in their own right, so all the fundamentals are there; good story telling, fleshed out characters, nicely paced, but I can't imagine the characters, well thought out as they are, being as compelling to someone who hasn't been with them for their previous adventures in the Long Earth. 
 Another reason this book may lack impact on a newcomer is that I found the biggest draw (for me personally, of course) was not what happened to Joshua, or Sally, or Maggie, but what they discovered. Don't get me wrong, I love the characters, but this novel could have been written with entirely new characters that had not featured before, and I'd still have been hooked on every page just for the sheer joy of discovering exactly what is at the ends of the Long Earth? What is on the many iterations of the Long Mars? What are the Next? 
 Practhett and Baxter have crafted a universe (universes?) that has me so engrossed in the sheer detail of how it works, what is in it, that the interesting tales that are being told are almost an added bonus. 
 If you have never read a Long Earth book, go back and read the others before reading this. If you have read the others and liked them (even if you weren't thrilled with the Long War), definitely read this book. 
 I do have one gripe about the story, but it's a spoiler, so I'd advise you only click in if you've read it... 
 (view spoiler)[The only part of this story that underwhelmed was the way the threads all wrapped up so quickly and easily. The Next, rescued from destruction at the hands of fearful Homo sapiens, simply leave humanity to their own devices, wishing to be left alone. The Armstrong II, having trekked through unimagined hostile worlds across most of the book, covers the return leg in little more than a page. Similarly, the arduous journey through the Long Mars achieves its goal at great cost, but again returns to the Gap and to Earth in a narrative breath. 
 Obviously Pratchett and Baxter are the best selling authors, so maybe there was no better way to handle things, but it felt a little rushed to me. Perhaps it would have been more understandable if The Long Mars was to be the last book in the series, but I understand there are another two planned, so it seems odd that the authors would be so keen to wrap things up neatly with a little bow in this novel. 
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