The Troublesome Apprentice — Liza O'Connor

While investigating the death of a friend and client, Maddy Hamilton, Xavier Thorn (reputed to be the greatest sleuth in England) is greatly impressed with Maddy's nephew, Victor, and offers him a job as his secretary. Aware of Xavier's history of firing secretaries, Victor garners a promise that for three months he cannot be fired.
Vic then proceeds, in Xavier's view, to be cheeky and impertinent at every turn. Xavier endures the impudent pup because Victor is most skilled in extracting the truth from clients and intuiting facts with little evidence to assist.
As they solve a string of cases, Xavier discovers a few more important details about his troublesome apprentice, such as her true gender, and the realization that she has awakened his long dormant heart.