You decide to change her back, and even use another spell to make him muscular again, since he seems to want that.
You open the Book of Body, point at the girl and speak the command word 'gender.'
Smoke appears around her, and when the smoke fades, she's a he again. Though, his hair for some reason seems to have stayed the same length.
You then look through the book for the other spell he wanted.
"Muscle." You speak, while pointing at him. Another cloud of smoke appaers, and when this one fades, he has much larger muscles than most normal people can hope to obtain. He resembled a professional body builder, if those exhist.
"So.... What is your name?" You ask him.
"Oh, it's John, and thanks for changing me back."
"No problem.... but you want revenge on them right?"
You explain about the person who has the Book of Mind, and how he's probaly somewhere in the mansion. How he's after the other books, and is apparently willing to kill others. Of course, you also mention how he's probaly the one who gave the order to take John's book in the first place. Then finally youi ask your question.
"I'm trying to find him and steal his book, though I'm not sure if I can do it alone. Even if his lackeys are rather dim witted, he himself seems to be more prepared. I'm trying to ask, for help."
He smiles. "I'd love to take a shot at the guy responsible. You can have his book. I'm going to crack his skull."
Well, at least John is easy to please.
"Alright then."
You stop and wonder if you should cast any more spells, or continue looking for a way in. You can think of a few that might be useful, but you don't want to waste energy if they're not needed.