TL;DR: Starts strong, interesting concept, great characters, but plot and science both begin to fall apart in the back third.
Micaiah Johnson takes a solid, interesting premise about as far as the artificial constraints placed around it allow in this satisfying, but flawed, book.
In an ill-defined future (could be 100 years from now or 500, can't really tell), an ecologically-ravaged earth (not even sure it's OUR Earth, honestly) is divided between walled, safe, secure cities and the wastelands without. Nothing original there, really. But a Steve Jobs/Elon Musk-ian tech visionary perfects a way to travel the multiverse, allowing "traversers" to visit some 300-odd "nearby" earths. The catches are: we can only reach earths that are very, very similar to ours, and the traverser must be dead on the destination world as the multiverse protects itself rather violently from the Grandfather Paradox.
That second catch provides the most interesting conceit of the book: the safe, secure, wealthy (read: white) people of the cities are generally not able to be traversers. That class must come from the wastelands, the sun-scorched (read: brown) poor folks who have nothing, who are most likely to NOT have survived in all of the other worlds reachable from this one, and who are therefore able to travel there to... well, that's not entirely explained very well at any point.
The bulk of the plot revolves around the protagonist's questions of identity on multiple layers; Cara's slightly different lives on each earth, which she is increasingly aware of as she makes more trips, how those lives intersect with the various others that are close to her in her "primary", ie, actually-lived life on earth, how THOSE folks are different (or not) in each other earth she visits...
The books works best when analyzing person-to-person relationships, and how much of our lives are circumscribed by circumstance and birth more than anything else. I don't think I can ever read another book that adheres to the American myth of rugged individualism being the prime mover in every person's life, so I'm glad this book doesn't treat with that nonsense at all.
Where the book does wobble, for me, is in the world-building... somewhat. The wasteland dwellers (Ashtowners), their enforcers have giant murder cars but... guns are not only outlawed, but the very knowledge of how to build them has been lost? Wiley City, where the rich folk live, has massive skyscrapers and full computer networks and can TRAVEL TO OTHER UNIVERSES but how to build a gun is lost to them?
I realize this was basically necessary for plot purposes, but man that conceit is hard to buy as the reader.
Likewise, I'm not sure why traversers have to exist... they're extremely rare, but also treated like Wal*Mart greeters by their employer. And that's because they... don't seem to do much? They pull data from a nebulously-described network of ports they've previously installed on the other earths. What anybody DOES with this data, or why it has any value, is never defined. While I enjoy how the employment status of the traversers is tied to their caste, effectively, and not their actual labor (much like how "Essential Workers" are not renumerated at all based on that essentialness on our Earth but, perversely, as little as possible instead), and the discussions that occur around this dissonance of labor, I almost have to side with the Big Tech Company here because the incredible labor described here is not tied to any actual produced value. It's... just weird.
That said, if you can swallow all of these little inconsistencies, the characters and their conflicts are engaging enough to keep you going even through the six-way car crash that is the plot resolution (the ending, which I'm being purposely vague about for spoiler purposes, does not gel at all for me in terms of how the plot is resolved, even if I still liked how the characters are resolved, in the end. If that makes sense).
I feel that, if Johnson can figure out how to put a little more internal consistency into her world-building and end-plotting, she's already nailed dialogue and characterization to such a high degree that writing a great book is in her grasp. Even with the jankiness of some elements, I cared about the main characters and wanted to know how they all turned out very much. She's good at getting the reader engaged emotionally with the leads. That said, the "sci" in this sci-fi feels like the bare minimum that was necessary to get her characters into the places she wanted them, and that framework just doesn't hold up in the end.