I'm reading some bits and sections of Spengler's Decline of the West for my senior thesis. His influence on Heidegger is apparent, to say the least. He makes quite a few remarks, as well, on the evolution of categorization that resemble arguments in Foucault's The Order of Things, including comments on Marx and Darwin and their coinciding epochal-making understanding of economics and biology. There's also quite a lot of off-the wall conclusions--on this point, it works well for my purposes.