I had an interview in Waterstones this week (though not the same branch, or I probs wouldn't be posting!). I only had one question even vaguely about books (although I am an English graduate, so maybe they assume that I'd know some stuff? idk) - and that was something about a book that has had a big impact on you. Can't quite remember.
Other questions I got were based a lot around things like team-work and customer service - there wasn't just one question on each of these things, but a block i.e. "what do you think makes good customer service?", "how do you know when you've given good customer service", "give an example of when you've gone out of your way to help a customer" etc. Also stuff about working with difficult people, dealing with difficult customers. Ooh, and one about how much supervision you were under in your last job, and whether that level suited you. Generally, they were quite typical interview questions (have you checked out the interview thread?).
Quite a few of my questions took the assumption that I'd had a job before ("give an example of a time when you've used your initiative to boost sales" - that kind of thing). Maybe they'd use a different set if you haven't? I suppose the thing to do would be do think about how you would hypothetically deal with those situations?
She told me at the start that everyone had the same questions - don't know if that meant everyone just at that branch. She was certainly reading them off some official looking form.
Most of all - don't panic. My interview certainly was v relaxed, and although I got a bit tongue-twisted on a couple of questions, I jut kinda laughed it off, took a moment and then clarified. No big deal. She was really friendly. It was maybe about 1/2hr, though I couldn't be sure.
I'd probably not go for the tie if I were you - a nice shirt and decent trousers should be fine. I didn't go super-formal. Don't really think it's that kind of place. As long as you look clean and presentable, and like you've made an effort it'll be fine.
(Also: I put my personal tutor down c/o the english office at my uni - but obviously he won't be contactable either. I'm sure they realise that it's the summer holidays. Did you put someone else down?)
Good luck!