This book was written concurrently with the author's international bestseller "Communion" and published just one year later. It turns out Strieber had always intended for his exploration of what happened to him with the visitors to become a publishing series, starting with "Communion" (which is genuinely worth reading) and continuing through further volumes, which sold far less well and are not so highly regarded.
Whereas "Communion" has a simple, visceral power and reads as a coherent, chronological narrative about the author's experiences with the visitors, "Transformation" is more a collection of recollections and speculations which didn't make it into the first book. It jumps from one idea to another and is less focussed than the first volume, though it does contain some interesting stuff. Further abductions both of Whitley and his son Andrew (whose real name is used in this book) are described, some of which do read as dream-like; their reality is degraded by the uncomfortable fact that, whereas thousands of abduction accounts from other abductees describe exactly the same beings and processes, Strieber's are in many details unique and different. This is usually a red flag, and lends support to the contention expressed by many that Strieber "has a hard time telling fact from fiction."
At the core of "Transformation" is the author's struggle to find the meaning of the experience, and here he goes way off the map. He journeys into metaphysical and rather new-age territory as he comes to believe the visitors recycle souls, and that the Earth is a kind of "school." There are echoes of Jim Sparks' writings in the self-indulgent, obsessively introspective narcissism which characterises much of the book's content.
Strieber is still a good writer though, and this is the saving grace of "Transformation." From the pen of a less competent writer it might read as a rambling incoherent mess, a meandering tract of new-age mumbo-jumbo. However the book is better than that: it's just nowhere near as good as "Communion" and doesn't make any lasting contribution to the understanding of what is going on with this pervasive phenomenon - though it pretends it does.
In summary, "Transformation" would be of interest mainly to the devotee of Strieber's writing (and he's admittedly a good writer), or to anyone who wants to read everything ever written about the abduction/visitor issue. It's OK, but not one of the better books on the subject.