The second Agents of the Crown book features James Sidwell, Marquis of Riverdale, whose spying has been curtailed because a French spy has exposed his identity. So while he has nothing much to do he's called down to his aunt's estate to help her identify a blackmailer. While in disguise as the new estate manager he meets Elizabeth Hotchkiss, companion to his aunt, well-bred but penniless.
Elizabeth has just stumbled on a book in her employer's library called 'How to Marry a Marquis.' Since marriage to someone wealthy seems to be the only way she can support her younger siblings, she gets trapped into trying out the edicts in the book – trying them on the only available male, James, who is, of course, a marquis in disguise.