How many books should I have read around my subject before I go to interview?

There is no set number of books to have read which will make you prepared for the interview for your subject. Particularly in Modern Languages, which is my area, reading books can only maybe help you with the vocabulary in the unseen text you're given as part of the interview process for analysis. It sucks how vague it seems, but I think you should see this as a weight/pressure lifted from your shoulders because you can't quantify the passion and interest in your subject in the number of books you've read.