"Can you help me find some mystery books?"
"Yes, I'd be happy to. What kind of mysteries are you looking for?"
"The kind where someone is killed, and then they figure out who did it."
In the mystery fiction section, I tried this. "There are different kinds of mysteries. Some are more gritty and violent. Some are more gentle. Some are humourous. Do you know what might interest you at all? Maybe you've seen a mystery on TV that you liked?"
He said, "You know, it makes you think that one person did it, but really it was another person?"
Not a lot to go on there! I pulled three different books by three different popular mystery authors, and he went happily on his way.
In case it seems like I'm making fun of this customer, I'm not at all. I thought it was very sweet. I give him credit for being able to ask for help on such a basic level. The interaction was an great reminder to not assume knowledge or use jargon. Your next customer could be a first-time reader.