The book concerns a rich old man named Johann Sebastian Bach Smith, who (in order to live longer), wishes to have his brain transplanted in a younger body. The twist comes in when it gets transplanted into the body of his sexy young female secretary! There's a lot of gender-bending and sometimes thought-provoking questions about male vs. female psyches, but after 500 pages, it gets rather lengthy and dull.
In the author's defense, Heinlein (as always) manages to paint some very interesting and charismatic personalities on paper. If you haven't yet read it (although most people have), I recommend instead Heinlein's Stranger in a Strange Land. While also oversexed, is much more interesting and not quite as droning as I Will Fear No Evil.
This is definitely a dialogue-driven book. After the protagonist wakes up and learns that he is in the body of his secretary, and that his secretary's mind is still in her body even though her brain was removed and his was implanted in its place, there is little of what might be called "action" except for a mild courtroom drama.
I've had little exposure to Heinlein; one of the first SF books I read was _Space Cadet_, which I enjoyed, although at ten I wasn't particularly discriminating. Two or three years ago I tried to read _Stranger in a Strange Land_, and was bored by it; I couldn't make it through to the end.
This book, however, was a completely different animal. The dialogue is compelling, and its frequently sexual nature challenges us to examine our notions of love and gender.
Near the end there is a surprise twist: Was Heinlein playing with our minds, leading us on, throughout the entire book? Is Joan/Johann crazy? Did s/he snap? When I finished the book I was inspired to read it again, immediately, to try and find an answer to this question.
This is one of the most intellectually stimulating, mentally engaging, and challenging science fiction books I've ever read. Judging by the reviews here, you either love it or hate it, and there isn't much middle ground.