FAREWELL SIDONIA by Erich Hackl, translated by Edna McCown (Fromm: $16.95; 123 pp.). Abandoned by her mother in 1933 Austria, Sidonia, a sickly Gypsy baby, is nursed to health by loving foster parents. Sidonia’s dark skin decides her fate as the Nazi movement sweeps over the Alps; people who are different, like Gypsies, are being rounded up and sent to concentration camps. Little Sidonia is an engaging, happy child. But the times are violent and the town is poor. As Sidonia confronts a merciless community, the idealism and the love of good, decent people are all she has in her favor. And no one, apparently, is responsible for what happens. This story is told so sparingly there is no time to build defenses. It has a power that is beyond tears.