Nightjohn by Gary Paulsen

From chapters.ca:
Sarny, a female slave at the Waller plantation, first sees Nightjohn when he is brought there with a rope around his neck, his body covered in scars. He had escaped north to freedom, but he came back - came back to teach reading. Knowing that the penalty for reading is dismemberment Nightjohn still retumed to slavery to teach others how to read. And twelve- year- old Sarny is willing to take the risk to learn.