Probably my favourite book that I've read so far this year, though the edition (which isn't available on goodreads) is in Czech and also contains Korsch's On Marxist Dialectic and On Materialist Dialectic. The main premise of the main text is a “call to action” for Marxists to return back to and emphasise the dialectic more, as to not fall into the trap of Marxism becoming a stagnant ideology, an “inverse thinking” as Korsch puts it. Even though the text might be more difficult to read at points, it displays the shortcomings of Marxism and Marxists that haven't properly understood Marx's dialectic in a very striking manner. It also clears up some of the fog that surrounds Marxism with it's relation to science and philosophy and tries to “put it back on the right course”, so to speak. Korsch, along with Lukács, who is also mentioned several times in the book, form an important baseline for the positive and negative critique of Marxists from a left position. These striking remarks and pitfalls of which Korsch warns of can be seen developed later in the Soviet Union, especially Marxism becoming something of a sacred text, an ideology, not a critique (of political economy) as Marx understood it.
Also “a Marxist critique of Marx” just sounds cool.