Give a referral, next thing you know the email back is "do you have his/her/its email/contact info"
Refer to a comparable title, the email back is "where can I buy it"
Yen Cheong's invaluable book publicity blog says it better than I can:
– Don't waste time by asking stupid questions.
What is a stupid question ? The definition of a stupid question is very simple: it's one you can pretty easily answer yourself. Kind of like, "Can you tell me who wrote this book?"
If you are asked a stupid question, you can either
1) say you don't answer stupid questions
2) answer the stupid question or
3) provide the link to the answer to the stupid question.
I suggest option 3.