When I was in Waterstone's today I overheard a guy ask one of the booksellers if she could recommend a book to him. Apparently he was looking for something along the lines of D.M. Thomas' "The White Hotel" or something by Ian McEwan - so presumably something fairly weird and depressing with lots of sex. Or as he put it "Dark and disturbing, but in a positive way." I'm not sure what she recommended, but he wandered off happily, bag in hand, while the booksellers fell about laughing.
However, it's still not as good as some of the requests Alasdair got in Waterstone's (correct me if I'm wrong Alasdair):
"It's about this size with a blue cover."
"How often do the annuals come out?"
And my personal favourite,
"Do you have any books on how to raise the dead?"