Lessons in Ruin by Avery Maitland

Constance Blackwood has had some luck with her delicate arrangements, but not everyone is pleased with the games she had been playing with the reputations of Queen Charlotte’s courtiers.
A match gone wrong could spell the end of Constance’s little assignments—and it is difficult not to suspect that her closest allies could be planning her downfall.
The Earl of Farraday’s only daughter has been unlucky in love, and now that her third season has just ended, the earl is desperate to see her married. But when an unexpected match made in the first blush of spring sets tempers alight, the Countess of Farraday will look to Constance for an explanation, and a quick marriage to ensure that their daughter’s honor remains intact.
Another problem weighs heavily on Constance’s mind. Her precious diary has been stolen, and whoever has taken it is in possession of information that could change the course of Constace’s fortunes and spell the end of everything she has worked so hard to build.