The Well-Tempered Clavier II was conceived as a part of Bach's ambitious project to compile many large scale works of different genre. In it we can confirm roughly one fourth of the collection being the revised work of his earlier pieces.
Bach compiled the WTC II between 1739 and 1742. Unlike the other contemporary pieces such as "Goldberg" Variations and the Art of Fugue, he perhaps never considered seriously to publish it in the form of a printed edition. Bach kept on revising the work until the final years of his life, suggesting that the work is an 'unfinished' product. Indeed, there survives neither the Fassung letzter Hand -- the definitive version of the autograph -- nor copies stemming from it. As far as we can trace, there were two main sources originating from the composer -- the one so-called 'London' autograph, and the other lost autograph set which can be reconstructed to certain extent from the surviving copies. Bach revised not only these two sets at different times but also the copies, the most notable of which was the one made by J. C. Altnickol in 1744. The complexity of source situation owes much to the absence of the Fassung letzter Hand, but we cannot underestimate the degree of its enormous popularity, which resulted in the production of numerous copies and numerous variants therein introduced by the later generations.
Here I intend to give a very brief outline of this intriguing topic by using MIDI files. If you require further technical information, please refer to my published volumes of Critical Commentary, Vol.1 and Vol.2. Further discussions on interpretations, please look out for my forthcoming book "The Genesis and Early History of Bach's Well-tempered Clavier, Book II: a composer and his editions, c.1738-1850", which should appear (hopefully) in 2006.