The Last Miss Lynton — Ann Edgeworth

Unlike her five older sisters, Miss Victoria Lynton is no beauty, but endowed instead with a lively mind and a tendency to outrageous behavior. Bored with being a debutante in fashionable Edwardian Society and intrigued by life below stairs, she disguises herself as a nursemaid, taking a job at a grand country house in Devon.
It is coincidence that among the house guests are Alex Beaumont, a most attractive man once glimpsed and never forgotten by Vicky, and his barrister friend, Thomas Craig. Coolly disapproving the latter is a surprisingly frequent visitor to the nursery. How can Vicky successfully balance her subterfuge downstairs with her intriguing relationships upstairs?