I have a great book called "A Primer of Drug Action", which is a textbook I used for my Psychopharmacology class in college. It is excellent and I think that, if your mom is smart and can understand the language, it might clarify the pharmaceutical facts versus the layperson myths about cannabis.

It also discusses the facts about other drugs.

It lists only the facts, not as pros and cons, but as medical facts, so the reader can make his or her own value judgment based on the facts presented.

I think it is very good but it might be a tough read if you don't have a college-level science education.

http://www.amazon.com/Primer-Drug-Ac.../dp/080507158X