She starts thinking about how hot having a truly perfect body will be and begins to instinctively begin to touch herself. Reaching down into her pants and feeling the wetness of her genitalia, and realizing that pretty soo she won't have that wetness, realizing that she's feeling something that won't exist anymore. She feels her clit, her urethra, even letting her fingers brush up agaisnt her anus, knowing that she soon wouldn't have those parts. It feels so hot knowing this is her last chance to feel the pleasure of having those parts of her body.
Then suddenly, as she rolls over with organic joy, she closes the book by laying on it. She didn't even get to cum one last time before she starts changing.
She feels pain and weakness drain from her body, as her body fat shrinks and muscle mass grows, giving her a slightly toned appearance. Her skin clears as body hair and blemishes dissappear into it. Then finally she feels her sexual organs go away. First her bladder and womb disappearing from inside her, then her holes sealing shut, and finally the skin around them smoothing over until her entire body is completely smooth in her bikini area, like a doll.