"I'LL ALWAYS THINK OF ELLA AS A young girl," Frank Sinatra said a few years ago. "Her pure, crystal-clear voice is music unto itself." This week (April 25) the owner of that beguiling voice celebrates her 75th birthday, an event that has prompted the release of two new retrospectives. Ella Fitzgerald: 75th Birthday Celebration (GRP) is a, two-CD set of her early work on Decca (1938-55). It includes her swing rendition of "A-Tisket A-Tasket," the 1938 hit with the Chick Webb big band that launched her career, and scat performances of "Flying Home" and "Oh, Lady Be Good," which established her as a pioneer vocal bebopper of the '40s. Ella Fitzgerald: First Lady of Song (Verve), a 3-CD set, draws from her classic "Song Book" albums of the '50s and '60s, as well as from sessions with Louis Armstrong, Count Basie, and Duke Ellington. Both sets showcase her inimitable virtues, which include a gossamer way with melody and the rhythmic dexterity of a tap dancer.