The political economy of flat taxes
Taxation was, by the way, always a key concern of classical political economy (it's there in the title of Mr. Ricardo's Principles). One of the most famous tax proposals of the 19th century, Henry George's single land tax, was in fact based on the 'Ricardian' theory of the rent. That is, the idea that landowners derive their income (rent) from ownership, and that their income (for a given level of output) detracts from profits and the possibilities of accumulation, required a tax to transfer income against wealthy landowners.
The interesting thing is that since the old classical surplus approach makes it clear that class conflict is essential for understanding the functioning of the economy, it does not try to disguise the issue of taxation as not having distributive consequences.