Well I finished and, as I expected, the book was very much left open for the next one. There is a lot of stuff not explained, but that's ok with me as this and the other books are very much about provoking thought.
I am reminded of classic Sci-Fi with this series. It's a sort of "what-if" series. They have introduced a premise - what if you could step sideways into infinite versions of Earth? From that idea they have then, I think, expanded on that idea to pose such questions as: What would this ability mean for Humanity and for the Datum Earth?
In the last book we were introduced to a terrorist attack on a US city, where people were able to step away to relative safety. In The Long Mars we have an enormous natural disaster that has rendered the Datum Earth pretty much uninhabitable in vast areas. The idea of the Yellowstone super-volcano causing such damage, we are told, is quite possible and maybe even likely. So the exploration of the effect this has is really quite well investigated. Of course those people in the book were able to step away from the disaster - we wouldn't be able to. In many ways it's the sort of reverse to what we get in a lot of post-disaster fiction. In this case one realises that as we don't have that ability to step away, the outcome for us could be pretty terrible. Normal dystopian fiction says: this is what things might be like if something dreadful happened. This book is, in many ways, more frightening because the threat is real and there is no escape.
Clearly we have not seen the last of the Next. I was sort of reminded of Arthur C Clarke's "Childhood's End" and John Wyndham's "The Midwich Cuckoos" with this story arc. It's not a new idea for a story, but I do like the way that this has been handled in this book. Of course the Next would see humans as apes. As single kids they are disturbed and frustrated. Picked on in school and bored at how simple things are to them. Once they get together as a group, and are able to communicate and swap ideas, they move even further away from homo sapiens. That increased strength would make trying to converse with ordinary people, incredibly slow and frustrating. Add to that the fact that those ordinary humans were prepared to wipe them out, must surely mean that we haven't seen the last of them. I suspect even Lobsang will have problems with them.
I'm still thinking about the Mars story. I found it interesting that Long Mars and Long Earth are not synchronized. The Mars in the sky of West 1 is not the same Mars as Mars West 1. This may make the idea of cheap easy space-flight using the Gap Earth, practically useless.
And then we have the Space Elevator technology. This again is an Arthur C Clarke idea. At least he has created it in his books - I don't know if it was his idea in the first place. What I want to know is how Sally's father managed to get a small strand of the cable. Was it just lying there waiting to be picked up? He obviously didn't cut it from the main cable. Not only would that be near impossible with any tools he might have, but it also be incredibly dangerous for a cable under that much tension to be cut.
So if space travel via the Gap Earth isn't going to be useful, maybe space elevators on the various Step Earths will.
I will look forward to the next book.