I can tell you that 'Fifty Shades of Grey' has sparked a whole subgenre of similar series. The original trilogy itself consists of three books: 'Fifty Shades of Grey,' 'Fifty Shades Darker,' and 'Fifty Shades Freed.' Beyond that, there are plenty of series with a similar vibe, like the 'Crossfire' series by Sylvia Day, which has five books, and the 'Bared to You' series, also by Sylvia Day, which spans four books.
Another notable mention is the 'This Man' trilogy by Jodi Ellen Malpas, which follows a similar theme of intense, passionate relationships with a dominant male lead. If you're into more suspenseful romance, the 'Stark Trilogy' by J. Kenner might be up your alley, with three books that blend steamy scenes with a gripping plot. For those who prefer a darker tone, the 'Monster in His Eyes' series by J.M. Darhower offers a three-book arc with a compelling antihero. Each of these series captures the essence of what made 'Fifty Shades' so addictive, with their own unique twists and turns.