These pieces deserve more love than history can give them. A series of chicaneries (read below if this sort of trifle interests you) relegates these extremely inventive and personal creations to a sort of annoying footnote in Busonology.
First, we are not faced with the sexiest title Busoni ever gave to his pianistic spawn. Literally, "Pieces for the development of Polyphonic Playing." Most pianists and music-lovers immediately ignore (for this superficial reason) what they believe to be a pre-modern dentistry handbook or Protestant morality tract !
Worse, there’s generally confusion about how many pieces to play. The number of pieces depends on the edition used: first published as five pieces, Busoni later appended a sixth (a very short preludietto) from the first edition (in five volumes) of his Klavierübung (another unsavory title: see article on this massive work). The second edition of the Klavierübung (in ten volumes, extremely rare, since Breitkopf and Härtel decided to continue publishing the first volume and discontinue the second, in spite of its fuller, definitive character) publishes seven pieces, adding a mysterious study for the third pedal of modern Steinway pianos.
This Busonologist prefers the six-piece layout for its division in Scarlatti-like pairs:
— A weird and perky little inversion study in E
[in E] — A very refined and strange "Trio Sonata" texture throughout
3. Andante molto tranquillo e legato
— Simply one of the oddest pieces of all times! A hypothesis for its floating, indecisive movement: Busoni was finally thinking of his abandoned project of third-tone chromaticism. We know he had ordered an expensive harmonium in this tuning system in the 1920’s. Could his ear have become accustomed to such a point that his semi-tone music was affected? Ends in A, marked attacca, so it is a clearly a prelude to what follows.
[in D] — A heroic, surging work with many canonic details. Anthony Beaumont used this searing music to complete his version of Faust’s final (unfinished at Busoni’s death) monologue in Doktor Faust.
[in G] (Andante tranquillo) — One of Busoni’s touching Chorale creations recalling, in yearning tones, the third Elegie or Albumblatt. The atmosphere here is transcendent and mystic, constantly slipping in and out of stable tonality.
6. Adagio nach Mozart
[in C minor] — A quite literal transcription of the "Armoured Men" music from Die Zauberflöte (also a Chorale). Busoni, as in his cycle An die Jugend, sees the future in the past, and vice-versa. He introduces a subtle, but quite dissonant variant in Mozart’s texture. The sonority created, ringing and dry, is unique in the piano literature.