"Hardcore readers" refers to people who are extremely dedicated and passionate about reading. They read books regularly and often have a deep love and enthusiasm for literature. Full definition
They are offering something that the competition is not doing and hope that a number of hardcore readers who buy a lot of e-books will flock to your ecosystem to save money. (goodereader.com)
Mostly, I think only hardcore readers would notice the difference, though! (self-pub.net)
You probably don't own an e-reader unless you're a pretty hardcore reader. (thecreativepenn.com)