New from Bach

HE isn't one of your newer composers: In three months we'll celebrate the 300 th anniversary of his birth. But Johann Sebastian Bach is producing his matchless organ music again, so to speak. Thirty-three Bach organ chorales, unknown for more than a century, have just been discovered. It is a Christmas gift to classical music lovers from one of history's greatest composers.

The pieces are relatively early works that show the composer developing musically, ''breaking away from his models . . . becoming harmonically much more adventurous,'' says Bach expert Christoph Wolff, who found the works. The discovery comes as the music of Bach and baroque-era colleagues is enjoying extraordinary popularity. The last similar musical event to command such attention was the authentication four years ago of a symphony by Wolfgang Amadeus Mozart, composed when he was nine years old.

Together the two discoveries point anew to the contribution of music historians. Not only do they add to our knowledge of musical masters and their works, but occasionally they also uncover previously unknown pieces by famed composers.