Humble Bundle, a promotional site that lets consumers pay what they wish for bundles of DRM-free content, continues to grow e-book revenue via its unusual business model. The site reported $11 million in e-book sales in 2016, up from $6.1 million in 2015. It attracts more than six million unique visits each month and has more than eight million customers and more than eight million email subscribers.

Kelley Allen, Humble Bundle’s director of e-books, said the company’s e-book bundles, which were added to the site in 2014, have had “fantastic success.” She added, “More and more new customers are coming to us for e-books, and we keep adding new genres.”

HB generates most of its sales from bundles of video games, but the site increased the number of e-book bundle promotions from 32 in 2015 to 53 bundles (including 20 focused on comics) in 2016. E-book revenue was 11% of total revenue last year. HB publisher partners include Dark Horse Comics, HarperCollins, IDW, Make, O’Reilly, Simon & Schuster, and Viz Media.

Since the launch of the site’s e-book portal, Humble Bundle has generated $30 million in revenue from 130 e-book bundles, an average of about $230,000 per bundle. The highest-grossing e-book bundles in 2016 include Pathfinder/Paizo’s RPG Book Bundle ($1.3 million total on 77,498 bundles); the No Starch Press Hacking book bundle ($979,059 total on 67,560 bundles); and the O’Reilly Unix bundle ($938,953 total on 64,703 bundles). The highest-revenue comics bundle was by Image Comics ($319,618 on 20,007 bundles). The Neil Gaiman book bundle released in late 2016 (a collection of stories, speeches, scripts, and other writings) generated $394,820 on 24,960 bundles.

The site also allows users to designate a portion of their donations for each bundle to go to a charity. In 2016, HB donated $16 million to various charities, and in 2017 HB launched the Freedom Bundle, which includes books, comics, games, and music, to protest the Trump travel ban; it raised $6.7 million. Sometime this year, the site will mark more than $100 million in donations to charity since its beginning in 2010.

Looking ahead, Allen said Humble Bundle is redesigning its website to be able to launch more e-book bundles, including textbook bundles, in addition to more technology bundles featuring titles on coding and the maker movement. She also wants more comics and role-playing games, and she wants more big publishers to join the program.

Indeed, Allen acknowledged a need to find other kinds of DRM options to attract certain publishers (usually big trade houses) that continue to insist the promotions contain DRM. HB has done one bundle with DRM, with DC Comics last year. “We need to find a less onerous form of DRM or develop one ourselves,” Allen said, though she noted that indie houses and small presses were happy to strip DRM from their titles.

Allen praised the addition of e-books to Humble Bundle. “By experimenting in a creative business model such as Humble Bundle, book and comics publishers of all sorts have the ability to thrive while in tandem contributing millions of dollars to charity,” she said. “It’s a win-win-win for publishers, readers, and charities.”