The Lesson, Stoney Ridge Seasons Series by Suzanne Woods Fisher and published by Revell.
In her wildest dreams, spunky and impulsive nineteen-year-old Mary Kate Lapp never imagined herself behind a schoolteacher's desk. A run-in (literally) with the schoolteacher compels her to act as a substitute teacher, just as her restless desire to see the world compels her to apply for a passport . . . just in case. The only thing of interest to M.K. in the sleepy Amish community of Stoney Ridge is the unexplained death of a sheep farmer that coincided with the arrival of a mysterious young man into the community. Frustrated that no one takes the crime seriously, she takes matters into her own hands. Unfortunately, as tends to be the case for M.K., she jumps headlong into trouble.