When Israeli author Orlev, who drew on his own ghetto experiences in The Island on Bird Street (1984; Batchelder Award), met a certain Polish journalist, they found that both had been boys in Warsaw during WW II; Orlev kept ``Marek's'' extensive confidences secret (including his discovery in 1942 that his father--executed in 1934 as a Communist--was Jewish) until his death in 1987. Now, Orlev shapes Marek's account into a powerful novel about a devout 13-year-old Catholic in a virulently anti-Semitic society, responding to his experiences by coming to champion the Jews walled in near his home. With stepfather Antony, Marek already knows the ghetto: traveling through sewers, they take food to sell there at high prices, often returning with a baby to hide with the nuns (no charge). Still, Marek is casually anti-Semitic until he helps rob a Jewish escapee and is caught by his mother, who points out that ``You sentenced him to death'' and reveals his own heritage. Deeply shaken, Marek sets out to make amends. He befriends a man he sees crossing himself the wrong way and ultimately leads him back, underground, to the ghetto, during the heroic ghetto uprising. Orlev's characters are sobering, believable blends: e.g., Antony dislikes Jews but, knowing Marek's background, wants to adopt him; he turns others' dire needs to profit but has ``nothing against human beings.'' Many others in this richly authentic story are equally complex. Subtle, beautifully crafted, altogether compelling. (Fiction. 10+) -- Copyright ©1991, Kirkus Associates, LP. All rights reserved.