The basic premise: a wealthy, crotchety old man contracts to have a highly experimental and illegal brain transplant performed to preserve his life; unfortunately, the first suitable donor is his much-adored, beautiful, and spunky secretary who is herself killed in a mugging. Typically good Heinleinian legal shennanegans follow as the sucessful transplantee must prove his identity, and then the meat of the novel (after many thousands of words of set-up) begins as he must adapt to being a "she."
Much of the novel from that point is an enjoyable speculative exploration of the idea of having ones consciousness transplanted to another, altogether different body, including passages where the female's "ghost" shares the young body's mind and the reborn old man finds himself attracted to, having a child with, and eventually sharing a body with his old business partner. The latter portions are somewhat below Heinlein's normal standards of tight, well-defined prose--but then again, during the writing of I Will Fear No Evil, Heinlein did suffer a long relapse of tuberculosis that frequently interrupted his writing efforts.
Heinlein novices would be advised to read Starship Troopers, Stranger in a Strange Land, The Moon Is a Harsh Mistress, and The Past Through Tomorrow (his "future history" anthology) first, but those already familiar with the grand master of sci-fi will likely find this still a fun read--and a requirement for the complete Heinlein library.