Pasternak letters charm and inform; The Correspondence of Boris Pasternak and Olga Friedenberg: 1910-1954, compiled and edited by Elliott Mossman. Translated by Elliott Mossman and Margaret Wettlin. San Diego: Harcourt Brace Jovanovich. 410 pp. $19.95.

When the young poet Pasternak first became acquainted with his dazzling cousin Olga, during their families' summer meetings at various resorts, he persuaded himself he was in love with her. But she deflected ''Borya's'' ardor into a loving friendship that spanned a turbulent half-century including the Russian Revolution and two World Wars.

Their letters describe his progress as an artist, hers as a scholar and teacher; they exhort each other to work harder, and exchange often heated literary and philosophical arguments. There's something charming, and quite touching about the way they try to be worthy of each other. And, inevitably, the darkness of the times colors their communication.

This rich book adds importantly to our understanding of Pasternak's artistic growth, and restores the memory of a brave, gifted woman who was in many ways (as she well knew) his equal.