She decided to take off her clothing for the transformation. Best to be able to see her body before it all happened, or even while it all happened. Looking at her breasts she realized how soon she wouldn't be seeing them at all, she wouldn't be seeing anything womanly about her body in awhile.
Feeling horny at the idea that this was the last time she'd be able to see herself like that, she decided to masturbate one last time. Taking her finger she started rubbing her clit, feeling how large it was made knowing how shrunken it would soon be. She started feeling the pleasure of being inside herself, knowing it was her last chance, her last chance even to be wet. She orgasmed, the sensation feeling better then ever before knowing her next orgasm was probably years away.
Then she shut the book. The first thing she felt was the glow of her orgasm unnaturally stop. The warmth and moistness of her genitals went away, and the thumping of her heart calmed. All signs she had just cum disappeared. Her vagina shifted from the bloomed flower it had been for years, to a small undeveloped slit, completely useless for anything sexual.
She felt her body follow suit. Her hips became boyish, and her breasts disappeared until she felt the odd feeling of a totally flat chest. Her skin became smooth, all of the body hair disappearing from her form and acne clearing up. Finally, she noticed sex toys and menstrual products that used to be lying around her apartment weren't there anymore, she didn't need them.
Looking at herself she saw a completely sexless person. An adult for sure, she certainly wasn't a child, but she was an adult yet to be sexually developed at all.