From Forbes (2018):
Film adaptations of books gross 44% more at the U.K. box office and a full 53% more worldwide than films from original screenplays, according to research commissioned by the Publishers Association and produced by Frontier Economics.
The report also found that 43% of the top 20 highest-grossing films in the U.K. from 2007 to 2016 were book-based and another 9% were based on comic books. Data for the report was compiled from a variety of sources, publishing industry magazine The Bookseller notes, incorporating case studies and publically available information alongside contributions from the BBC, UK Theatre and Nielsen BookScan.
“In short, published material is the basis of 52% of top U.K. films in the last 10 years, and accounts for an even higher share of revenue from these leading performers, at 61% of U.K. box office gross and 65% of worldwide gross,” says the report, adding later that “Across any of the common measures of viewership, book adaptations on average outperform shows based on original scripts or on comic books and other sources.”
Link to the rest at Forbes