Michael J Sullivan is a new author to me - an Amazon recommendation when I was hunting for Kindle/Audible pairings. He's achieved trad published and self-published books - unusually this one was crowd funded, which allowed him to pay for professional external editing and audiobook production under his own control.
In many ways that has worked well - the narrator (Jonathan Davis) is good and the text he's reading is sound, editing-wise. Infodumps were few and far between. So the mechanics are good.
What about the story, then? It took me on an interesting journey, with a sympathetic, but far-from-perfect protagonist, Ellis Rogers. Ellis grows along the way, experiences loss and healing, learns to outgrow his prejudices and in most ways becomes a better human being. He also saves the world.
The book is reminiscent of Heinlein's The Door into Summer, which I intend as a compliment. Except that there isn't a cat, and Ellis is, if anything, a dog person. Oh well...
Hollow World is themed around time travel, which is Sullivan's primary "what-if". Unlike The Door into Summer, it's one way. That moves it into the Rip Van Winkle territory, so perhaps the what-if is "here's humanity, two thousand years in the future. It's different."
Ellis Rogers builds a time machine in his garage, is diagnosed with an incurable disease, and pushes the button. Arriving not quite when he expected, he stumbles straight into a murder, then meets Pax, whom we realise pretty quickly is going to be a major character.
Pax is an arbitrator, so by my reckoning ought to be able to persuade people. However, his main technique seems to be to say "X is true. I can't tell you how I know. Trust me."
This happens often enough that it becomes obvious that Sullivan is hiding a key plot element from the reader. For me, that breaks the rules of mystery writing - you don't point to the locked box and say "the solution lies within, but I the author am just going to tantalise you until it suits me to reveal what I have concealed therein." For the sake of balance, I will admit that he does provide clues as to what might be in the box (and I guessed right). But a good mystery novel works by hiding the truth in plain sight, then surrounding it with misdirection. Sleight of hand, not stonewalling.
I said I guessed right about the contents of the metaphorical box containing the key plot element. I'm afraid that it was a disappointment, in that it added yet another what-if, which breaks the rules of sci-fi - you make one major change to reality only.
Pax says "I can't tell you how I know - trust me" once too often, loses the trust of Ellis Rogers, and vanishes. Thereafter Ellis is on his own, taken in by the plausible lies of those about him, while I the reader am seriously considering a DNF on this, despite the excellent world-building and a lovable-but-annoying AI. The villains are too obvious, and Ellis' stubborn refusal to believe ill of his friend became seriously irritating. Again, there's lots of hand-waving by the author, and you know he's hiding stuff.
Finally the nature of the plot is revealed and it turns out that Hollow World has a single point of failure - really? They have Maker technology that can transmute elements and turn out pre-charged batteries of near-limitless power - and yet the villains decide to employ a twentieth-century device as their weapon - really? And that business of teleporting (yet another what-if) the weapons out into interstellar space? Umm, you're in the core of the planet, surrounded by molten rock - just dropping the things into the molten rock is quite sufficient to render the most powerful of weapons quite ineffective - if you don't believe me, ask Frodo.
Finally, I have to believe that Sullivan's tongue was firmly in cheek when he engineered a thrilling race against time, and maker-equipped Ellis with a digital watch so we could have a countdown in the movie version.
There is good in the book - the world-building - and bad - the physics - and ugly - the breaking of literary conventions. So I did finish the book, and it did take me with Ellis on a journey to a pat ending.
But the stuff I had to put up with along the way...