THE RIGHTEOUS The Unsung Heroes of the Holocaust By Martin Gilbert Illustrated. 529 pages. Henry Holt. $35. -------------------------------------------------------------------------------- Hitler intended to exterminate European Jews without exception, and he almost succeeded. Nevertheless, thousands survived in Nazi-occupied Europe, hiding or using false identities. They could not have survived without the help of non-Jews; the helpers were a minority but not an insubstantial one. Some Jews hid in the sewers of big cities, others in isolated farmhouses; some found refuge in forests, others in convents. Help was extended even by courageous inmates inside Nazi camps, which often made the difference between survival and...