I just bought the book on my lone wolf playthrough and broke my bank doing so. I really hope they hot fix this soon since my fiance and I are going to be in Nibelheim very soon on our main playthrough. I have no idea why the book would disappear like that after you first talk to her.
I haven't encountered Victoria yet and don't know what the book is for but when I find her should I make sure to buy it immediately if I can afford it? How much is it without barter/charisma scores?
I noticed a price difference on the two playthroughs, but not sure what effected them. The first one (where I was stupid and didn't grab the book because I had so little gold) was around 1200. The second playthrough I paid around 952 I think it was. I would suggest stealing a few paintings around town and buying the book as quickly as you can - while you can.
I'm still trying to kill time before I head to the area where I need the book, but we'll probably be there later tonight or tomorrow. :\