Highlander in Disguise by Julia London

GRIFFIN LOCKHART HOLDS THE KEY TO HIS FAMILY'S FATE. SINCE HIS BROTHER LIAM FAILED TO RECLAIM THE PRICELESS HEIRLOOM THAT COULD SAVE THEIR ANCESTRAL SCOTTISH ESTATE, IT'S NOW UP TO GRIF TO FIND IT --- AMONG THE LORDS AND LADIES OF FASHIONABLE LONDON SOCIETY.

Disguised as a Scottish earl, Grif attends the most glittering balls, hunting for the woman who is rumored to possess his family's treasure. Along the way he catches the eye of Anna Addison, a highborn young woman whose sharp tongue and even sharper wits have limited her marital prospects but enable her to detect Grif's deception. Determined to find a husband this Season, Anna draws Grif into a scandalous bargain: She will deliver his precious heirloom --- and keep secret his true identity --- if Grif can teach her how to seduce a man and win his heart. Well aware of what a man wants from a woman, Grif reluctantly instructs her. Soon Anna is besieged by suitors and Grif's exasperation with the troublesome beauty turns into heated desire. With time running out, Grif commits a reckless act in order to claim not only his treasure, but the passionate woman he believes is his and his alone.

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