Comprising twenty-one chorale preludes as well as the four duets, and opening and closing with the mighty 'St Anne' Prelude and Fugue, the third book of Bach's Clavier-Übung is so much more than its somewhat prosaic title ('Keyboard Practice') might suggest. Jeremy Filsell proves the ideal guide, adroitly matching each piece to one of the five organs of St Thomas's in New York.