Many readers might still be unaware that there are a lot of free e-books that publishers quietly give away online, and it's taking place on a site called BookBub.

Described as the "Groupon for e-books" by The Guardian, the site specializes in sending out daily emails to alert its members of free or discounted e-books.

In 2013, Random House distributed free e-books of their best-selling Dan Brown novel, "The Da Vinci Code" for a week, according to The Book Insider. The publishing company wanted to drum up excitement for the release of Dan Brown's latest novel, "Inferno," and used the site to market free e-books that included the first chapter.

There has been a spate of deals for BookBub members since. "I now have more books than I can read in a lifetime," a site member, Suzie Miller, told The Book Insider.

Launched in 2012 by Nicholas M. Ciarelli and Josh Schanker, BookBub has since grown to two million subscribers after a year, according to Publisher's Weekly. The site's rapid growth has its members to thank for the word-of-mouth, but BookBub is also known to target its audience base with an expert editorial team that carefully curates titles that would will click with its members, and arranges to have these e-books available at discounted prices.

BookBub separates the titles by categories, so subscribers can subscribe to a particular list based on their interest. Of late, it's the mysteries and contemporary romance categories that are getting more listings, according to Publisher's Weekly.

It's the list that not only publishers would like to get on; self-publishing and previously rejected authors are keen to market their novels using BookBub's resources, according to author Mike Alvear via Huffington Post. However, Alvear also shares that it's not easy for authors to get on the list as they reject almost 80-90 percent of submissions.

"There's a good reason for this. They don't want to paralyze their subscribers with too many choices. They know if subscribers trust them to only market a few stellar books at a time, they have a better chance of keeping that subscriber happy and making money off them," Alvear wrote.