This book is about much more than just one’s internal monologue. It does tackle that, but also the many non-verbal rabbit-holes that our brains can easily disappear into.
The author is an experimental psychologist, and brings his professional knowledge and experience to bear on this problem—citing many studies, including his own studies from his own lab, in which he undertook to answer precisely the implicit questions of “How can I…” in terms of tackling these matters, from root anxiety (for example) to end-state executive dysfunction (for example).
The writing style isn’t dense science though, and is very approachable for all.
The greatest value in this book lies in its prescriptive element, that is to say, its advice, especially in the category of evidence-based things we can do to improve matters for ourselves; beyond generic things like “mindfulness-based stress reduction” to much more specific things like “observe yourself in the 3rd person for a moment” and “take a break to imagine looking back on this later” and “interrupt yourself with a brief manual task”. With these sorts of interventions and more, we can shift the voice in our head from critic to coach.
Bottom line: if you would like your brain to let you get on with the things you actually want to do instead of constantly sidetracking you, this is the book for you.