That is the new, interesting, and engaging book by Sean Carroll. Some of it is exposition, the rest argues for a version of Many Worlds Theory, but with a finite number of universes. Here is one excerpt:
String theory, loop quantum gravity, and other ideas share a common pattern: they start with a set of classical variables, then quantize. From the perspective we’ve been following in this book, that’s a little backward. Nature is quantum from the start, described by a wave function evolving according to an appropriate version of the Schroedinger equation. Things like “space” and “fields” and “particles” are useful ways of talking about that wave function in a appropriate classical limit. We don’t want to start with space and fields and quantize them; we want to extract them from an intrinsically quantum wave function.
I very much liked the discussion on pp.300-301 on how a finite number of quantum degrees of freedom implies a finite-dimensional Hilbert space for the system as a whole, that in turn constraining the number of worlds in an Everett-like model. If only I understood it properly…
You can buy the book here.