The dirty book is a once off item Pixie doesn't sell.
If you've talk to her the day after giving Emmi the fantasy book, she sells it to you through dialogue (provided you have 3K gold).
I don't know what happens if you don't have the necessary gold, but if she doesn't give you the option to buy it when you go to her again, I assume you'll probably have to buy another fantasy book, redo event, and then redo Pixie's dialogue to get it.