I actually did read the book, mainly for educational purposes. My theory on its popularity is that it is extreme enough to stir and excite people, yet it doesn't go too far as that would scare them away.
Story of O by Pauline Reage and
Justine by de Sade make it look like a Sunday school picnic. Most of people in the BDSM community absolutely hate this book, too.
My other theory on its popularity is that it glorifies gold-digging and also glorifies dishonesty about it. The woman is a college senior who is a virgin when she meets him. She's been "too good" for a relationship, then she suddenly gives into this guy. She also claims that she's not interested in his wallet.
The man is just a pathetic loser who throws money around trying to impress the woman. When she shows up at the airport, she learns that he has paid for an upgrade to first class. He also gives her an old rare and collectible book.
It's hard to find a story with two more unlikeable characters.
What can be learned? Well, you can learn how to write a crappy book that sells a lot. Many complain that it repeats the same words over and over. The fact is that this has a hypnotic effect and gets people into it.
Here is a funny article by Dave Barry on the book:
http://ideas.time.com/2014/03/04/dave-ba...s-of-grey/
Finally, we men should remember what Ellsworth Toohey says in
The Fountainhead: "Never bother to examine a folly, ask only what it accomplishes." If the book makes a woman in your life horny, make the most of it.,