Philosophical Literature
In Marx's day, economics was known as political economy. It combined a science- and math-y vibe (that's the economy part) with headstrong ideas about how society should function politically (that's the political part).
This two-in-one combo often made the subject more philosophical than typical economics literature today turns out to be. But political economy's got a practical bent, too: the focus is on the real world of labor, not just head-in-the-clouds ideas. So Das Kapital is philosophical literature, but about the real-world concerns of labor.