>>24716754 (OP)
The collected works of Marx and Engels. Deceptively simple to read most of the time. But make an attempt to create a cohesive collection of their ideas and suddenly the task is quite difficult, especially as you need a background in Hegel to understand the more abstract points and Hegel’s a beast to read. Marx’s more involved works also have a lot of context in the science, economics and history of his day, so you need to know that as well.