1) Orgelbuchlein, BWV 599-644 by Johann Sebastian Bach. This is a wonderful collection by many respects. It is not easy, because most of the 46 chorale preludes have 4 independent voices, including a pedal line. However, the technical challenges are compensated by the very short length of each piece - most of them are only 1 page long in the autograph. They are perfect as a preparation for longer chorale preludes.
2) 44 organ chorales by Johann Christoph Bach. This is very practical and playable collection of the composer from the Bach family. The chorales are written in the fughetta form with points of imitation. Most of the pieces contain very easy pedal part. which also could be played on the manual. Highly recommended for organists with modest technical abilities.
3) Chorale preludes by Johann Pachelbel. Excellent works by the significant south German Baroque composer. Compositions are written using variety of compositional techniques, most notably vorimitation and cantus firmus with long note values in the soprano voice. Most of the works employ 4 voices and many of these pieces can also be played on manuals only.
4) Chorales from the Clavierubung by Johann Ludwig Krebs. This is a superb collection by one of the most famous student of J.S.Bach and contains 13 chorales each written in the following system: a prelude (praeambulum), manualiter chorale with chorale melody in one of the voices and chorale harmonization with the soprano and bass part in continuo notation. The last part could be played with or without pedals.
5) Chorale partitas by Johann Pachelbel. These are wonderful sets of manualiter variations in 2, 3, and 4 voices. Each variation employs one melodic and rhythmic figuration throughout. They are highly practical and playable by organists with modest technical abilities. In addition, they are perfect for demonstration of variety of organ stops and their combinations.
Note that this list is not written in a graded order. Although these pieces don't require an advanced organ technique, they might not be sight-readable. Some organists will have to put in some practice time in order to master them but they are artistically very pleasing and well worth the effort. They will be perfect works for service playing and recitals.
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