Chater's thesis is simple: while your mind feels deep—with a rich subconscious—it's not at all. In fact, it's shockingly flat. There is no hidden "true" self, you aren't unconsciously processing and thinking, and introspection is imagination.
He doesn't expect to convince or convert you immediately, so the book slowly chips away at ideas of a deep mind using modern neuroscience and cognitive psychology. Did he convince me? Sort of, or at least, not "no". It's a book that has offers an interesting perspective, whether or not it's "right" about everything. I see this as more of highlighting the complexities of mind, and pointing at truths that may be missed by an overly-bold assumption about consciousness. This book pairs incredibly well with Focusing and IFS / parts work books, because they both present useful models that mechanistically disagree, but join together to present an interesting and fascinating view of the mind.