For the innovative and fascinating theory of mind and world it propounds, this gets five stars despite some underdeveloped features of the system. Minus one for its repetitious and under-edited format (the book is a collection of previously-published articles with overlapping content; alas, for this reason I can't in good conscience give it a full five stars as I would if it were cohesively constructed). So four stars total. :) Note: YOU SHOULD READ THIS BOOK, BUT NOT AS YOUR FIRST ACADEMIC PHIL OF MIND BOOK. For maximum results, you need to have at least a basic acquaintance with the core positions and arguments in analytic ontology and philosophy of mind before you read this book. But then, for the love of God, give Kastrup a fair hearing. If his work gets taken as seriously as it deserves, it has the potential to shake up the lazy-minded prejudices of the contemporary philosophical scene, and perhaps even to bring about the sea-change in Western thought that its author envisions.
Let me first note with satisfaction that Kastrup takes the Hard Problem of Consciousness (hereafter "the hard problem") extremely seriously. I take it extremely seriously too. Not everyone does. I don't claim to be able to argue compellingly for taking it that seriously. But notwithstanding that disclaimer, it just seems obvious to me that the hard problem is devastating to a standard physicalist view in which consciousness is not fundamental. It's not just an epistemic problem; it's a metaphysical problem, and it is insuperable. Physicalists who treat it as just a bullet to be bitten or something to be explained away, puzzling to be sure but potentially outweighed by the merits of physicalism, are simply failing to realize how huge a problem it is. The only way they could fail to see this is that they're emotionally, socially, and professionally attached to physicalism, or simply lack the mental clarity to reflect on their basic experiences without contortions of intellectual self-deceit. All extant physicalist lines on the hard problem are pure hand-waving; to think that new standard-physicalist solutions will do any better is to miss the point of the hard problem. Hence, either "the physical" is consciousness-involving, or consciousness is not physical. Kastrup, like me, finds panpsychism (he doesn't consider panprotopsychism) to be an unconvincing band-aid. Thus we are left with the conclusion that consciousness is not physical. Dualism is better than its reputation: the interaction problem is WAY less serious than the hard problem; the causal closure principle is just a principle; the paradoxes of epiphenomenalism could perhaps be resolved. But dualism is also messy and unparsimonious, and it's not the only or even the best non-physicalist game in town.
Where do we go from here? Kastrup opts to build a system that assigns ontological primacy what we apodictically know: conscious experience. He argues for a careful distinction between being conscious of something and KNOWING that you are conscious of that thing (bye-bye higher-order theories), another point whose obvious correctness could only be obscured by pure bias. He reminds us that the existence of "matter" outside us is not some unassailable, known fact (sorry, Moore), but rather a theoretical posit (or, I would amend, in some ways a pre-theoretical assumption, but no more justified for that). Kastrup's system, which is rough-hewn as yet but in some ways quite powerful, shows that belief in external "matter" is by no means abductively inescapable either.
The system itself is a form of idealism in which the one genuine existent is universal consciousness. The appearance of separate conscious subjects arises from a dissociation of some contents of universal consciousness from other contents; this is why I can't read your thoughts. Why did this dissociation occur? The book doesn't say, and Kastrup when questioned tends to favor a teleological explanation (as do I). I'm not sure what the explanation is in his mind, but perhaps it's along these lines: universal consciousness wanted to enrich its experience, explore its own essence, in some way that is only possible through dissociation. (Incidentally, this view offers a solution to the problem of evil. If I intentionally stub my toe and then ingest a drug that makes me lose my memory and experience myself AS that toe, the problem of evil might occur to this "thinking toe," but no one would be at fault.) But Kastrup is surely correct that much of the intuitive appeal of the "external, material world," and the intuitive mistrust of idealism, stems from cultural indoctrination into materialism. (My own experiences have suggested to me that materialism is, in a sense, the mouthpiece of the ego's fears and illusions; the materialist transmutes those fears into beliefs as a spurious defense against them, as though by allying himself with his destroyer he could avoid destruction. Kastrup has a different, also plausible explanation for why materialism is a defense mechanism.)
I believe it is a major point in favor of Kastrup's idealism that it harmonizes with the world's mystical traditions. It also offers explanations of empirical facts about mind-brain correlations, psychedelic drugs, and quantum mechanics, although I freely confess that I am unqualified to judge how successfully it copes with the last of these.
I also believe the system has some flaws. This is OK. Kastrup is swimming against the current here; although there is a revival of panpsychism going on, and some glimmerings of interest in idealism, his is practically a lone voice in the wilderness. He hasn't had the benefit of modern, analytic-philosophical debate, collaboration, and refinement of a fully sophisticated idealism (or, better yet, idealisms, plural). In a just world, Kastrup's efforts would form a starting point for a brand-new school of neo-idealism to hone and refine. The pessimist in me doubts that academic philosophy will get there anytime soon, but one has to hope!
To me the main weakness of the system is its account of the body. To Kastrup, the physical body is the visible form of dissociated consciousness, viewed from across a dissociative boundary. He thinks this for reasons similar to Schopenhauer's: our body is not just another "external" object, but one to which we have intimate access. The unifying principle of this body is metabolism, which surprisingly turns out to ground the dissociation itself. I think this is a strange view. First, the boundaries of the body actually are somewhat vague. Second, it's not clear to me why a paradigmatically non-"mental" process like metabolism can ground something as radical as mental dissociation. Perhaps it can, but Kastrup needs to do more work to show that this is plausible. I would suggest that cutting my finger is no more "internal" to my essential subjectivity than breaking the wine-glass I am holding. The former is sensed via nerve impulse, the latter by sight; why should one of these be more intimately "part of me" than the other?
I also believe that the problem of apparent unconsciousness deserves a more robust response, although one may not be possible at the current state of our knowledge. If I am essentially conscious, then why do I seem to go unconscious in deep sleep, under general anesthesia, or after a blow to the head? Yes, it is possible that these are not genuine instances of unconsciousness, but I'd like to see a better reply to this objection than a mere claim of possibility.
Finally, I am uncomfortable (intellectually and emotionally) with Kastrup's account of the mental life of God, or mind-at-large. It is a picture inflected by Schopenhauer's doctrine of the universal Will: an instinctive force without metacognition. Perhaps I've got this wrong, but this seems to suggest that God is a kind of dumb animal, and humans are smarter than It. In any case, the view that universal consciousness lacks metacognition is arguably contradicted by NDE reports and some mystical experiences. Or perhaps there is no metacognition in It, but mystics have consistently reported It to be characterized by an intelligence infinitely greater than the human mind. Finally, Kastrup's own preference for teleological explanations of Its actions would seem better harmonized with a metacognitively aware universal mind (albeit not impossible on a non-metacognitive version). So I think Kastrup could do better at harmonizing his picture of the universal mind with the available testimony. Simply put, it's hard to see how his God is worthy of our deepest reverence, let alone worship. Since he takes the restoration of meaning to the world to be one of the main payoffs of his system, this is an urgent problem.
I could say a lot more, but there's no need. I'll close by reiterating that you should absolutely read this book. It deserves attention. It deserves debate. It deserves not to be ignored by an unreflective, lazy materialist mainstream. It will expand your mind. You will see the world in a new way.