The price of an obscure, out-of-print history book has skyrocketed in the last few days—thanks to Elon Musk.

The 45-year-old Tesla chief executive recently said he was reading William Bolitho’s Twelve Against the Gods, describing it “really quite good.” Bolitho’s book explores the lives of 12 prominent figures from history—including Alexander the Great, Casanova, and Christopher Columbus—and how they overcame the conventional thinking of their time. The description of the book reads:

This book is intended to elucidate history somewhat, more to illustrate it, to honor without hypocrisy the deeds of men and women whose destiny was larger, if not deeper than our own. Above all to shake loose the perception of the adventurer in us and of us in the adventurer

Since then, the price of a secondhand paperback edition of has jumped from $6.35 to $99.99 on Amazon’s US site, Bloomberg reported. Rare-book seller AbeBooks only had thirteen copies at the start of Wednesday and has since sold out.

“When we have less than 15 copies of a particular book then it’s safe to say that it’s pretty obscure,”AbeBooks spokesperson Richard Davies told The Guardian. “We had not sold a single copy this year until Musk revealed he was reading it.”

Musk—who co-founded PayPal before developing electric-car company Tesla, owns private-rocket firm SpaceX, and is trying to build a mass-transit system called the Hyperloop—is a well-known avid reader. When asked where he got the knowledge to build rockets, Musk once said, “I read books.”