How to Marry a Marquis by Julia Quinn

She's trying to follow the rules...

When Elizabeth Hotchkiss stumbles upon a copy of How to Marry a Marquis in her employer's library, she's convinced someone is playing a cruel joke. With three younger siblings to support, she knows she has to marry for money, but who might have guessed how desperate she's become? A guidebook to seduction might be just the thing she needs--and what harm could there be in taking a little peek?

...But he's making his own

James Sidwell, the Marquis of Riverdale, has been summoned to rescue his aunt from a blackmailer, a task that requires him to pose as the new estate manager, and his primary suspect is his aunt's companion, Elizabeth. Intrigued by the alluring young woman with the curious little rulebook, he gallantly offers to help her find a husband... by practicing her wiles on him. But when practice becomes all too perfect, James decides that there is only one rule worth following--that Elizabeth marry her marquis.

This book is part of the "Agents of the Crown" series.
Here are some other books from this series: