I think that David Moore's "fair old" instincts are not so rusty. The "Kassenbuch" is the cash book (or journal). So I would suggest that this is a creative merging of that term with "doppelte Buchhaltung". It is one of those times when the sum is greater (or rather, more nebulous) than the whole of its parts, meant to impress and leave us to ponder just what the author meant, while we wonder why it is we don't already know it.