A few years ago, Rebecca Yarros almost quit writing. A chronic illness often left her dizzy and exhausted, making it hard to work or even stand at times. She wondered if the stress she was putting on herself was worth it.

Then, she had the idea for a sprawling epic: a romance set at a military academy for dragon riders. The first novel, “Fourth Wing,” became an instant best seller, as did its sequel, “Iron Flame.”

Now, with the release of the third novel in the series, “Onyx Storm,” this month, Yarros has hit a new sales record. The book sold more than 2.7 million copies in its first week. All together, the three novels, part of Yarros’ planned five-book Empyrean series, have sold more than 12 million editions in the United States, according to her publisher, Entangled.

“It doesn’t feel real — none of it does,” Yarros told a packed auditorium of around 1,700 fans who had come to see her in St. Paul, Minn., on Wednesday night.

Print sales alone well exceeded a million copies in the novel’s first week, making “Onyx Storm” the fastest-selling adult fiction title of the past 20 years, according to Circana BookScan, which tracks print sales.