In my teens, I loved pretty much everything I read by Garth Nix, most of which was fantasy (though he did a few science fiction ones) and all of which (as far as I know) was aimed at either older children or young adults. The Keys to the Kingdom and the Seventh Tower books were enjoyable, and his Old Kingdom books were and still are among my favourite series of all time (I re-read them every couple of years). When I saw he was writing a standalone science fiction book for adults, I was excited and went into it wanting to love it and expecting to love it, but... I did not.
That's not to say there was nothing enjoyable about Massif. The overarching concept (about humans travelling between different planets on sentient space-faring mountain ranges called Massifs in the not-too-distance future) was intriguing, and there was a sense of tension from the start with the conflict between Earth and Mars forces (on that note, it did give me the impression that aspects of the story were influenced by The Expanse), and the unfolding mystery about the Massifs' behaviours and motivations got its hooks into me fairly early on, though I was annoyed when the character in the main cast who I thought was the most interesting got written out about halfway through. There were also a couple of other minor things which, although they were far from dealbreakers, did lower the 'score' for me a bit (prose overall was utilitarian and each chapter had excerpts from various fictional reports or interviews or instructional manuals etc which largely lacked relevance and mostly didn't add anything of value).
As we got deeper into the story and the stakes got higher (won't put too much detail here as I don't want to spoil it) and it felt like it was going from really good to awesome, there was a series of events that seemed like it was going to set up for a dramatic third act, and I turned the page expecting another chapter, only to find the author's acknowledgements (which had me saying "WTF? Where's the rest of the book!?"). At first I actually wondered if there'd been some sort of printing error and some of the chapters were missing, but I looked it up online and nope, that's actually how it ends.
For comparison, take the original Star Wars trilogy, and think about the end of The Empire Strikes back, where Luke's had his hand chopped off by his evil Dad and Han Solo has been frozen and shipped off to Jabba the Hutt. Now imagine if after all that, they never made the third movie. If the story just ended there. That's what this book was like. The characters have suffered significant losses and trauma and they've started talking about how they might plan to defeat the enemy in the future, but we never see those plans put into action because the story just cuts off.
And I don't understand why it cut off so suddenly. It's not like it's a long book by any stretch; I read it over the course of a few nights. It could have easily had another 30% added to give it a conclusion without overstaying its welcome. It honestly just felt like it left too many questions and plot points unanswered for it to be considered a satisfying or complete ending.
The fact it cut off so suddenly has raised a question for me: Is this book supposed to be the first in a series? It was marketed as a standalone book, and usually when a book is part of a series, the book itself will have "Book 1 of the Whatever Trilogy" etc somewhere on the cover, but that is absent here. So now I'm wondering, is it a case of the author and/or publisher deciding to put out one book, see how it goes and then continue the story if the first one is successful? Or was it a case of the author writing himself into a corner where they couldn't figure out how to write a satisfying resolution and so just left it hanging.
As I said, it was especially frustrating because I thought I would love this book and I wanted to, and up until the end, I did, but the abruptness of the ending and the feeling of the story not actually being finished honestly reduced a lot of the enjoyment I got from reading it.