By copying and filtering the music of others through his mind, he created his own way of doing things, which in turn made him an excellent teacher.
The preludes and fugues of the Well Tempered Clavier were written and used to instruct and entertain students and musicians. Leave it to the creative urge of Bach to write not just one set of 24 preludes and fugues, but two. But the differences in the two books are evidence that Bach didn't repeat himself with the second set.
The second book of the Well Tempered Clavier appeared roughly twenty years after the first volume, and Bach surely did not remain static. His style broadened, he encompassed more of the current trends in composing. While it can be said that the first book is more obviously geared to instruction, the second book is not as clear cut.
Prelude and Fugue No. 1 In C Major, BWV 870 - As in the first prelude of Book I, this prelude emphasizes harmonic progressions. But within those progressions occur snippets of melodies and themes, examples of how Bach could weave harmony and counterpoint into very satisfying music that makes profound musical sense.
The 3-voice fugue that follows has a subject that is 4 bars long, with a rest in the middle of it. Next the mildly declamatory prelude that precedes it, the fugue has a little bit of rhythmic bounce.
Prelude and Fugue No. 2 In C Minor, BWV 871 - C minor has been a key of passion and drama to many composers, and Bach wrote a dramatic prelude/ toccata in C minor in the first book. This prelude is decidedly less so. Any drama it has isn't obvious, and it is almost entirely written in two parts.
At only 28 bars, this fugue is somewhat short on the page. The subject is but one measure long, and Bach works out the fugue in a simpler form.
Prelude and Fugue No. 3 In C-sharp Major, BWV 872 - The only music Bach wrote in the key of C-sharp major is contained within the Well Tempered Clavier. The prelude of the first book is a brilliant piece, while this one is more studied and introverted and sounds akin to the C major prelude in the first book. There is a shift from 4/4 time to 3/8 time near the end and the music becomes a short fugato.
This 3-voiced fugue has a subject of only 5 notes, with the second entry coming before the first statement of it is complete.
Prelude and Fugue No. 4 In C-sharp Minor, BWV 873 - There was not always a particular feeling or emotion Bach conveyed with specific keys, but the key of C-sharp minor seems to be one of them. As in the first book, this prelude has a feeling of sadness. It is written in 3 voices throughout.
The fugue is in contrast to the prelude. It is written in 12/16 time, a compound meter of 4/4 time that implies a quick tempo. There is an interesting chromatic section within it.
Prelude and Fugue No. 5 In D Major, BWV 874 - The prelude opens with a fanfare and proceeds like a dance from one of Bach's sets of dance pieces. The first section is repeated, rather like a Scarlatti sonata, and some have conjectured that Bach knew about the development of sonata form. The theme bounces around the voices, and is answered as it makes its way to the end.
The 4-voiced fugue is stately and refined.
Prelude and Fugue No.6 In D Minor, BWV 875 - Written in two parts, a brilliant companion piece to the D minor prelude of Book One.
An interesting subject with a chromatic section in the eighth notes, and a complex set of note values from sixteenth triplets, sixteenth notes, and eighth notes.