This is a book by Neal Stephenson and Nicole Galland. I’m a huge fan of Neal. It’s safe to say he’s one of my favorite fiction authors. I liked this book, but I didn’t love it as much as some other Stephenson books.
The first irritant was some little detail problems. For example, on p670 the number "31142" is quoted as the number of square miles needed to search for a fugitive who can run 100 miles. That may seem overly nerdy to pick on, but this is a Neal Stephenson book.
On p647 we are told about some "large" vikings. Not large with respect to other vikings, but large with respect to modern people. That smells wrong to me since I have walked through many art museums and have seen thousands of suits of armor and historical garments. The fact is that finding Vikings who would seem large and imposing to modern Americans would be very hard in my opinion. Neal, with his LARP hobby (which makes an obligatory appearance), should know that.
And of course there is this rule of fiction writing which I just made up: if you do not want your plot to get into a muddle, do not write about time travel! Of course, Neal is a special writer and if someone is going to pull that off, he’s probably going to do it as well as it can be done. I think he did a great job with it, but I’m still convinced that it is more trouble than it’s worth. Neal and/or Nicole took the approach of using a combination of two popular fiction genres, Harry Potter and quantum mechanics esoterica. In the story there are magic-using witches and their magic has something to do with parallel universes as described by some of the weirder branches of quantum mechanics. (I always like to say that in some other universe, quantum mechanics has a better explanation.)
Ok, so there’s time travel. It’s explained away about as well as the plot of "Interstellar". I’ll suspend disbelief to be a good sport. So what do our main characters do with this power? They concoct the most byzantine (literally going to Byzantium to explain that word) Rube Goldberg missions. It just seems like such a clever author could have thought of less complicated ways to use time travel to, say, make money. For instance, instead of trying to go back in time and plant a mundane contemporary item that is very valuable today, why not just go back to some known treasure loss and figure out exactly where the lost galleon sunk?
Oh well, best not to worry about the details. In fact, with respect to some of the inevitable time travel plot muddle, p725 says, "..don’t think about it too much." Indeed.