Edition: New English Library, 1972
This long, latish Heinlein novel is a below-standard example of this period of his writing. It has an interesting, classic, science-fiction premise: it is the story of a rich old man who arranges an experimental brain transplant to try and avoid death, only to discover himself waking up in the body of a beautiful young woman. The theme of human interaction with, and response to, the challenges of new technology is at the very heart of much science fiction, particularly from the immediate post-war period when Heinlein really began to be a major force in the SF world.
The problem with this book is not with the theme but with Heinlein's treatment of it. He has the attitude toward sex of a twelve year old boy, and this seriously distorts the motivation of the main character. He brings in, without any justification, an idea of the survival of the mind which makes it possible for the transplanted brain to have internal conversations with the mind of the woman whose body it now inhabits. These conversations really highlight the paucity of Heinlein's ideas about human character. Basically, the woman goes around having sex with anyone she can get hold of, and the mind of the former occupant reassures the new mind that this is the normal state of affairs.
Bad as things are, they get worse when her new husband is killed, and his mind joins the two of them...
It would help if there was at least some attempt at a justification of some of this, but none is given at all. I quite enjoyed it when I first read it, when I was about 13, but re-reading it now has persuaded me to donate it to the nearest charity shop.