Good Night, Mr. Tom by Michelle Magorian

London is poised on the brink of World War II. Timid, scrawny Willie Beech -- the abused child of a single mother -- is evacuated to the English countryside. At first, he is terrified of everything, of the country sounds and sights, even of Mr. Tom, the gruff, kindly old man who has taken him in. But gradually Willie forgets the hate and despair of his past. He learns to love a world he never knew existed, a world of friendship and affection in which harsh words and daily beatings have no place. Then a telegram comes. Willie must return to his mother in London. When weeks pass by with no word from Willie, Mr. Tom sets out for London to look for the young boy he has come to love as a son.