They are "Bekker" page, column, and line numbers. Each Bekker page had two columns, hence 'a' and 'b,' and each page had roughly 35 lines.
They are THE STANDARD WAY TO REFER TO ARISTOTLE. Do not ever refer to Aristotle by using the particular page numbers at the top or bottom of the pages of your edition: use the marginal Bekker numbers.

The individual books do not add up to a coherent treatise.
That is because they were never intended to be one coherent treatise. The Politics that we know is comprised of several separate treatises that were juxtaposed into one work by a 1st century CE editor! Three centuries after Aristotle lived.

Aristotle is parsimonious: he says a lot with a little.
He takes small steps, but covers a great deal of material.
He uses words consistently: those who like that sort of thing call it 'technical terminology': those who don't call it 'jargon.'