You decide to try the gender changing spell. You wonder sometimes what you'd have been like as a girl instead of a boy, and this seems like a fun use for the spell as well as a good test. Wanting to see its effects, you get out of your pajamas and stand naked in front of your bedroom mirror, then quietly say 'gender'.
An odd sensation shoots through you, and in your head you see something- a bunch of sperm surround an egg. One breaks in, and then time reverses. This time, another one beats it, and it proceeds to fertilize the egg and form a xygote. You realize what this is- impregnation by a different sperm. The first time you were conceived, the sperm had a Y chromosome; this time, it has an X chromosome.
The magic reverberates through your existence and starts to catch up to your body in the present. Your skin smooths all over, and you see your figure turn curvy rather than straight. Your muscles turn from firm to lithe, and your hands and feet shrink slightly. Your shoulders narrow and your hips widen. Your face shifts around, as you develop softer cheeks, puffier lips, a narrower neck and a less sharp jawline.
Now the changes move to your sex characteristics. Your nipples spike in sensitivity and swell up, and you feel a desire to have them played with which is much more intense than you're used to. As you rub them curiously, you grow more and more aroused, and by the time the flesh on your chest starts accumulating into breast tissue, you're starting to feel willing to relinquish your manhood. If this is womanhood, it just feels better.
Only the penis and testicles between your legs remain of the young man you were before this spell, and with the male hormones in you having almost entirely transformed into female ones, it finally gives way. The shaft recedes into you even as it feels hard and erect, as your penis reshapes into your clitoris. You see your testicles pull up inside you, one after the other, and feel something indescribable as they turn into your ovaries. You are castrated, emasculated, you don't feel male anymore. But instead you feel female, and that feels way more right to you now. With satisfaction, you feel your vaginal tract and womb form inside you, and see your scrotum sink into you and form vaginal lips at its centre.
You stare at yourself in the mirror for a moment, amazed. The book's spell had turned you into a girl, with a body so perfectly feminized there was no hint you'd been born a boy. You wondered what to do next...