I was hooked because the first chapter was about Charles Bonnet Syndrome - CBS - and featured a lot of case histories about folks with macular degeneration or other visual impairment. Both of which ARMD and CBS were experienced by my mother before she died 3 years ago. CBS is when your visual cortex fills in the void when stimulation from the retina ceases to deliver. My mother saw both jagged patterns of a cartoon TV gone wonk and more explicit, exotic and detailed visions of specific ships and explicit sailors. Both are characteristic of CBS and making a named diagnosis was a relief for those who cared for / about my Mum - she wasn't losing her marbles entirely. What was interesting, and again this is characteristic, was how matter-of-fact she was about the loony-tunes film that was by-passing her ragged retina on a daily basis.
Another peculiarity of the syndrome is that the madey-uppy visions generated by the visual cortex don't have to make sense as / when interpreted by the higher, integrative, centres of the brain. Seemingly CBS folks, not especially serious musicians, are entertained by musical staves . . . but the melody is typically unplayable [I dunno, requiring six fingers on each hand?]. That's interesting because it tells us that the visual cortex knows nuffink about music and is really just pretending. Musical staves are rather an exotic point of focus but again that's typical of CBS - if you're pretending, might as well go full monty with flowers, Siamese princesses and knights with scimitars.
The rest of the book riffs on the extraordinary dance which links the processing brain with the sensory system of inputs. It can only really be understood when something goes wrong and bizarre things outfall. Hallucinations are not only visual but have be auditory - Jean d'Arc hearing the voice of god - or olfactory - believing you can smell chocolate pudding. Or, perhaps most distressingly, when you feel things which aren't there: remember phantom limbs on The Blob back in 2014?
Elsewhere in the book Sacks covers with much more authority hypnopompic and hypnagogic "dreams" and phantom limbs which have also been touched on in the Blobopast. It's also interestingly confessional, in that Sacks was out of his gourd on a crazy research program involving a combination of recreational drugs when they were still legal while he was a young medic in 1960s California. Interesting book: recommended.