The Debutante Is Mine by Vivienne Lorret

It's been awhile since I read a story with a dastardly cousin. And while Lilah's cousin mainly wanders into the periphery of this story, he's still there, all greasy and evil.

Yeah, so Ms. Lorret does a good job of explaining why Lilah has to be dictated by her father's will. Even though this is a trope we've seen often, there's at least some better explanation in this book other than just "Yep. This is the way it is." In order to fulfill her father's wishes, she has to marry a nobleman within three years or be forced to marry her cousin. The issue? This is her third season and  as a certified wallflower who most men don't remember, it's looking like marrying her cousin is a foregone conclusion. Then her eyes meet those of a dashing man on a large horse.
Jack Marlowe is the bastard (but acknowledged!) son of an earl who has had to fight for his opportunities in life, up to and including building his own massive personal fortune. At the end of the last book, his friend asked him to send flowers to Lilah in the hopes that it will help make her more popular and thus help her find a husband. Once Jack meets her though, he suspects that the duke, especially with his last warning away from Lilah, was really pushing them together. Especially since the duke is said to have developed a theorem to help people find their one true love.
There were some themes that I thought were underdeveloped in this book, like why Lilah felt so attached to the tenants on her family's land and what even happened to them. Also, what might keep Lilah and Jack together in the end. And the last Jack "twist" felt a little unnecessary. However, overall, the book built nicely on the first in the series and I'm looking forward to seeing what the backstory is for their friends' feud and how it's going to be resolved.