There were at least 4 books about D&D/RPGs 20 years ago:
What is Dungeons & Dragons? by John Butterfield et al
Dicing with Dragons by Ian Livingstone
Through Dungeons Deep by Robert Plamondon
Fantasy Role Playing Games by J. Eric Holmes
The latter was written by a psychiatrist. It is mainly an intro to the hobby but also addresses the nonsense that D&D was dangerous that was peddled by certain groups and mindlessly repeated by the media. The book is long OOP but it's not too hard to find a used copy.
And you can bet that I'll be getting D&D for Dummies. Within certain criteria, I have every book about RPGs and gamers.