Arguably they are made of two cycles -- 1007-9 and 10-12, the latter exploring further chordal writing, technique and scordatura. But my real reason for posting is to ask a couple of questions to the cello suite mavens here. First, what tradition do these suites belong to? Viol music? Or was there an existing repertoire of chordal solo cello music?
And second, is there something intrinsically "cellistic" about this music? Something important lost when played on viol or on viola or bass?
There had been music for solo cello and violin before: Domenico Gabrielli for the cello and Heinrich Biber for the violin. Bach though...took it where it hadn't been.
I think the cello cycle is fairly consistent though and I don't see a clear dividing line between groups. I think it follows a kind of emotional arc as well. It does seem to be more or less progressive in difficulty. Isserlis has hypothesized (maybe fancifully, but in ways it makes sense) that the whole cycle is meant to depict the life of Christ:
"Perhaps I should admit here that I too have a 'theory' about the story behind the suites, as I wrote in the sleeve-notes for my recording. I believe that they represent the life of Christ, with the 5th Suite portraying the Crucifixion, the 6th the Resurrection. I have absolutely no evidence for this - it is really a feeling, not a theory, in fact; but I do find it an inspiring vision".