Disclaimer: I am by no means a quantum physicist (or any type of physicist for that matter), the topic underpinning most of this book, and so my commentary should be read as a layperson trying to understand the arguments of the book rather than a critique of the accuracy of the arguments themselves.
The book opens by describing Young’s double slit experiment and its somewhat counterintuitive result due to wave interference, and then goes on to explain the same interference pattern is seen when a single photon is sent through the experiment at a time. Dr. Deutsch then argues that the single particle we can detect is being interfered with a real something, as opposed to something abstract like probability, that is evidence of an otherwise unseen multiverse which interacts directly with the universe we can observe. (The Khan Academy has a good explanation of the double slit experiment here, and two videos of the single photon experiment can be found here and here. If you really want to geek out on this here is a roughly 50 minute from Nobel laureate Richard Feynman on the topic.)
Deutsch then goes on to weave the aforementioned “Many Worlds” theory with inductivism, quantum computing, and Darwinian evolution to a rough framework for what reality is, assuming the things posited in the book were true. At times the subject matter was way over my head, but sticking through it led me to new ways to think about the world and its potential. One of the fascinating thoughts I took from the book was that what appears as junk DNA would be random in closely related universes and as a result would form a type of biological “crystal” that would stand out against the randomness of a single universe.
I recommend the book to anyone who is curious about how the world works and is willing to engage with something likely to be difficult to interpret at times to the point where it poses more questions than answers. Those questions are really what matters though, aren’t they? The two things I would point out as negative are that the book relied on an acceptance of what seemed like assertions and that the four “strands” of ideas were not clearly interwoven.