We all know about Bach's Well Tempered Clavier which we refer to as "book 1" of the "48". However, I have never seen Bach himself refer to the two books as being one glorious set of 48 preludes and fugues. As far as I know, Nach called book 2 simply 24 new preludes and fugues, and never called it "well tempered clavier". It definitely suggests to be a book 1 and 2 structure as the two sets are very similar in layout, but do you guys have any evidence (direct, no speculation please) that Bach intended the 48 as one "well tempered clavier" work?
An amateur Austrian musician named Philip Goeth has a website dedicated to studying the Well-Tempered Clavier works.
JS Bach completed the first book in 1722 when he was "Hofkapellmeister" at the court in Köthen, while the second book (which technically has not been named "Well-tempered Clavier book II" by JSB) was completed much later when he was holding the post of "Thomaskantor" in Leipzig. The 2 books thus result from two quite different periods of JSBs life, which is reflected in the character of the 2 books.
The Wikipedia article on Bach says that the second book was completed in 1744.
However, the Wikipedia article on the Well-Tempered Clavier says that the two books were only circulated as hand-copied manuscripts in Bach's time, and were not properly published, or mass-produced, until 1801, which is 51 years after J. S. Bach died. I think it is fair to say that however the two books were presented or packaged for the public, by that time neither J. S. Bach nor any of his family had any say in the matter. I suspect that we call the two volumes one set because that is how the publishing companies marketed them.