He smiled as he thought of a fun change. He had always felt bad about how people were so defined by their sex- even if they were a different gender and did what they could to be accepted as that gender, there would always be ways their body was that it had to be or couldn't be. Other species can change their sex so that they were just as indisputably male or female as those born male or female, so why shouldn't humans be that way too?
Picking up a pencil, he wrote on the page: "Humans are hermaphroditic and can change sex at will. If they desire to be female while in a male body, their Y chromosome will alter into an X and change their body to be female; if they desire to be male while in a female body, their second X chromosome will alter into a Y and change their body to be male. The functioning of their hormones and sex characteristics adapts to their sex and if they are fertile in one form, they will stay fertile in the other."
Once he put the pencil down, a tingling sensation seemed to emanate from the book out into the world, and somehow he could tell that the wish had been granted. "Wow, is it really that simple?" he asked himself. He could tell the answer was probably yes, but he had to know for sure. And so he focused on the desire he'd just described: he allowed himself to want to be female.
As he did, the tingling sensation returned- this time, inside him. His body felt warm and malleable. His clothes felt constraining against his shifting skin, and he undressed rapidly. He pulled off his T-shirt and could see his chest had turned smooth, and saw his figure slink inwards. He removed his jeans and his hips widened while his legs had softer flesh. He dropped his pants and saw his paradoxically hard penis strain and shiver in anticipation of the changes.
Turning his eyes to his naked reflection, he now saw his body turn to her body. His face seemed to resculpt from a man's to a woman's- the same person, but the opposite sex. He felt tension build in his chest and saw his nipples turn erect and double in size, before having soft flesh sprout around them as breasts formed. Expansion outwards on his chest was then paralleled by contraction inwards at his crotch, as his penis rapidly shrank away to a tiny sensitive clitoris, his testicles moved up inside the body to form ovaries and his scrotum sank inwards until it split down the middle to form a vagina.
As the transformation finished, she had to brace herself on the sink in amazement at the indescribable sensations of having his body emasculated and feminized. So the book did work, and it had made it so people could change sex whenever they liked! She wondered what to do in this totally different world...