Chicago a Cappella is the latest addition to the city's burgeoning collection of excellent unaccompanied choruses. Only three years after its founding this versatile nine-member group is about to release its first CD and plans a subscription season for 1996. Its strengths include well-schooled voices (the roster features respected soprano Carol Loverde, alto Terry Sullivan, and baritone Robert Heitzinger) and a wide range of musical idioms and performing styles. As coached by Jonathan Miller, a musicologist who sings bass, their voices blend to produce an exquisitely unaffected sound. The focus of this yuletide concert, "Mystery, Light & Joy," is Palestrina's music for the holidays, excerpted from the group's debut CD (on Centaur Records); the group believes this is the first time the three Palestrina pieces will be performed locally. The prolific Roman choirmaster's mass "O Magnum Mysterium," a minor achievement in the High Renaissance liturgical style, is a prime example of his fondness for flowing lines and for weaving ethereal textures from blended voices. Also on the bill are a pair of his polyphonic hymns, which should contrast neatly with their contemporary counterparts by Zoltan Kodaly and Franz Biebl, as well as spirituals--another indication of the group's catholic tastes. Saturday, 8 PM, Unity Temple, 875 Lake, Oak Park; 708-383-7599 or 800-746-4969.