The Beekeeper's Apprentice by Laurie R. King
In 1915, Sherlock Holmes is retired and quietly engaged in the study of honeybees when a young woman literally stumbles into him on the Sussex Downs. Fifteen years old, gawky, egotistical, and recently orphaned, the young Mary Russell displays an intellect to impress even Sherlock Holmes - and match him wit for wit. Under his reluctant tutelage, this very modern twentieth-century woman proves a deft protégée and a fitting partner for the Victorian detective. In their first case together, they must track down a kidnapped American senator's daughter and confront a truly cunning adversary - a bomber who has set trip wires for the sleuths and who will stop at nothing to end their partnership.