Author Tiffany Reisz subscribes to "the erotica writer's creed: It's not erotica until someone gets hurt." While I disagree with her blanket generalization, she definitely put her principles into action in The Siren, the first book of her Original Sinners series. The book was recommended by six of the fifty sources I consulted as a Fifty Shades of Grey readalike, and in this case I ended up disagreeing. Mild spoilers follow.
Nora still makes a significant amount of money from her A-list customers, a fact she continues to hide from Zach even as they grow more intimate. She wants to establish herself as a legitimate author so she can get out of the game, but there are still many things tying her to that community. Foremost among them is her complicated past with Søren, which she is mining for the new book. Along for the ride and attracted despite himself, Zach gets an education in the darker aspects of sexuality. While Zach struggles with his attraction to Nora and his lingering feelings for his wife, Nora tries to balance her feelings for Wes, Zach, and Søren, as well as finish her book by the deadline.
One of the reasons that The Siren is not a good readalike for Fifty Shades of Grey is because of the proliferation of love interests--Nora and Zach do spend some time paired together, but their relationship does not end in the happily ever after that I believe a 50 Shades fan would expect. A repeated theme in the book is that of lovers who are meant for each other but incompatible in some fundamental way and can't truly be together. The series continues through quite a few books, venturing off in different directions and focusing on previously minor characters in a way that does not suggest straightforward romance along the lines of what a typical 50 Shades fan would want. The narration is also not first-person, and shifts between Zach's perspective and Nora's.
Nora is a far cry from the virginal and innocent Anastasia Steele. In addition, the dominant male character in The Siren--Søren--is mostly a sinister offstage presence, and it is Nora that takes center stage. With all of Nora's manic, seductive, tormented, clever aspects (she is very clearly the titular "siren"), Reisz offers a more deeply realized female character than is often found in typical erotic romances. However, the characterization is such that it's hard not to wonder if Nora represents a rather extreme version of the author herself.
I don't think I'll be reading any farther, but I might recommend The Siren to a patron who is looking for erotic romance of a different flavor, especially one featuring a more experienced female lead. With a warning about the seduction of minors and the relatively heavy BDSM elements that Reisz explores.