The Door in the Wall — Marguerite de Angeli

Ever since he can remember, Robin, son of Sir John de Bureford, has been told what is expected of him as the son of a nobleman. He must learn the ways of knighthood. But Robin's destiny is changed in one stroke: He falls ill and loses the use of his legs. Fearing a plague, his servants abandon him and Robin is left alone.

A monk named Brother Luke rescues Robin and takes him to the hospice of St. Mark's, where he is taught woodcarving and -- much harder -- patience and strength. Says Brother Luke, "Thou hast only to follow the wall far enough and there will be a door in it?"

Robin soon enough learns what Brother Luke means. And when the great castle of Lindsay is in danger, it is Robin, who cannot mount a horse and ride to battle, who saves the townspeople and discovers there is more than one way to serve his king.