This book resists all efforts to define and pin it down. I would say that essentially this is a story of a man and his troubled relationship with the people around him, in particular his lover Glacia, and most importantly his self. But there are elements and themes that one might well find in SF or fantasy.
He attempts to define himself by writing down his past. But he is dissatisfied and rewrites it, each time becoming more abstract, more inventive until he constructs a fully imagined world but feels that it better incapsulates a deeper truth about his life. The book is split between two narratives both from the perspective of the protagonist Peter Sinclair, one set in the "real" world, the other in the "imagined" world and in each he has written his autobiographical manuscript but which is set in the other
An important theme in this book is the notion of identity and what makes us who we are. Do our memories define us? Give us the sense of continuity that allows us to go on thinking we are who we are? Would you be the same person if you had amnesia but you rebuilt your memories afterward from an autobiography you had written?
My only criticism of this book is that some parts of the narrative felt belaboured. But that is only a very minor criticism really. A very good book and I look forward to reading more by this author.
[EDIT:] I have revised my rating upward from four to five stars. After thinking about it for a few months, I am forced to accept that this is a masterpiece.