Have they ever smoked? Maybe you should ask them to be honest with you about their reasons for not wanting you to smoke.

Okay and if not... there is a good book:
Amazon.com: A Primer of Drug Action: A Concise, Nontechnical Guide to the Actions, Uses, and Side Effects of Psychoactive Drugs (Primer of Drug Action: A Concise, ... to the Actions, Uses, & Side Effects of): Books: Robert M. Julien
It has technical language in it, so it is primarily for people with college-level science knowledge, but it is very good and very unbiased in its reporting. It lists both positive and negative medical consequences of drug use, and based on the information, you can make up your own mind. It was a real eye-opener for me and changed my mind about cannabis. I used to be anti-drug but once I saw the facts for myself, I was more open-minded about it, and eventually tried it for myself.

This book is about the history and social usage of cannabis:
Amazon.com: Cannabis: A History: Books: Martin Booth