There's no gray area when it comes to the best-selling books of the decade.

According to The NPD Group, a market research company which collects data on book sales, E.L. James' erotic "Fifty Shades" trilogy marks the United States' three top-grossing books of the last 10 years.

The series, which includes “Fifty Shades of Grey,” “Fifty Shades Darker” and “Fifty Shades Freed,” sold 35 million print and e-book copies from 2011-19 and inspired film adaptations.

Written by the British author Erika Leonard under a pseudonym, the first “Fifty Shades” novel originated as fan-fiction of Stephanie Meyer’s “Twilight” series and was self-published as an e-book before becoming a fan favorite. The provocative trilogy chronicles a sexual relationship between a businessman and a college student. Despite their popularity, the novels are controversial and have been criticized for their portrayal of BDSM, a variety of sexual practices that include bondage, dominance and submission and sadomasochism. In Macaé, Brazil, for example, a judge ruled in January 2013 that bookstores throughout the city must either remove the series entirely from their shelves or ensure that the books are wrapped and placed out of the reach of minors.

All the books in the Top 10 list for the decade, except two, were authored by women.

"The Hunger Games" and "The Help” tied for fourth and fifth place on the list of best-selling books of the decade — each selling 8.7 million copies in that time. Paula Hawkins' 2015 "The Girl on The Train" placed sixth after selling 8.2 million copies and Gillian Flynn's 2012 thriller "Gone Girl," which sold 8.1 million copies, placed seventh.

The book list, announced Wednesday, also reflected the growing trend of book-to-screen adaptations. Nearly all of the top books of the decade were brought to the silver screen, including "The Fault in Our Stars" by John Green, "The Girl with The Dragon Tattoo" by Stieg Larsson, and "Divergent" by Veronica Roth, which rounded out the Top 10.