If a work is older than 200 years and worth reading, then original editions are going to be valuable.

But it will also be out of copyright so the cost of getting a “new” copy is basically just the cost of printing.

This was a 1970s paperback by someone who attracted attention for his work on spiritual matters and sold a lot of books but didn't leave an organization behind so you can find his books at used bookstores.

https://www.amazon.com/Discovering-Secrets-Happiness-Intimat...

Not rare at all but some people might say it has some prurient interest (talks about his sexual misadjustment) so maybe they think it has to be limited access or maybe people will steal it or something. (The same library kept Steal this book in a restricted area of the stacks but let me check it out.)

Likely a different edition, or reproduction.