I know both of the authors, Watterson better, so this is not an uninterested review. That said, I was there when they met.
The Mysteries, the new book from Watterson and Kascht exists partly due to a meeting between cartoonists brought together by Nick Galifianakis to work on The Art of Richard Thompson book. Kascht was asked to write a chapter on Richard's caricature, and met Bill at Richard's house. See a lovely video on that project here.
Overall, I wasn't sure what to make of this book. I'm still not, but I've just watched the creation video at https://www.youtube.com/watch?app=des... which helps me understand the mindsets of the two creators.
I have been surprised by the lack of buzz on the ground. The middle Tennessee Books-a-Million store I bought this in had it shelved in graphic novels. I think that it will eventually migrate to being a children's book. It's got the format of short text blocks and facing pictures, and it's a fable. I read it as a climate change fable, assuming that the mysteries are the natural world eventually interpreted through a scientific and industrial bent, until we reach the modern world.
The book isn't cartooning - the book is post-cartooning, it's what you do after you've reached the pinnacle of cartooning / caricature and are looking for something creatively new. This is exactly the type of thing you see in other cartoonists through the years - they turn to sculpture or painting, because they've said what they wanted to in comics. Daumier, Oliphant, Rube Goldberg, Bob Staake, Paul Conrad, and others turned to sculpture like Kascht; others to painting such as Jimmy Swinnerton, and like Watterson has tried pursuing at least 3 times (including Team Cul de Sac's donated painting and now this book).
That said, it's a minor book from two great artists. They made a video showing how they made the book, and how contentious it was to collaborate between them. The book stands on its own, but if they have to make a video explaining the book, then it doesn't work on one level, right? I'm glad they made it and maybe it'll break some creative block for both of them, but it's not going to be the most-cited work for either of them. Don't buy it looking for Calvin and Hobbes - buy it for its own merits, and to see what two crack cartoonists get up to when cartooning isn't enough for them anymore.