"It Depends," by Joseph Dietzgen
Extremely disappointed in this work, especially because it was allegedly so central according to so many out-of-context sources. People cite Marx’s letter where he called Dietzgen “our philosopher” but leave out the ones where he complained to Engels (and where Engels did the same) that this book could have been a much simpler, more concise pamphlet. Marx likewise (correctly) complained that it was clear Dietzgen had not read Hegel. In the introduction Dietzgen mentions a book about Hegel and how his philosophy was “outdated,” but never explains how or why except to assume that “speculative” means what it seems at face value (the opposite of his “scientific” approach he tries to describe in this work).
There are traces of Aristotle, Hume, Feuerbach, and a whole lot of Kant (though by his misuse of the word "transcendental," I assume he read a secondary summary). Unfortunately, not even the Kantian system is clearly worked out. Dietzgen at multiple points mixed up the faculty of Understanding with the faculty of Reason, saying that the former sought “general laws,” which was actually something Kant clearly explained worked in the latter. Dietzgen says that things are contradictory and then rails against contradiction. He can't decide whether he stands with Kant or Hegel because, well, it seems he barely read either.
The most important element of this book is that it represents the first concentrated attempt to understand epistemology from a socialist perspective. He goes beyond Proudhon, Kant, Engels, Feuerbach, Marx in his attempt at elucidating the role of consciousness from a materialist point of view. His relation-based view of reality is admirable, as is his view that thoughts, ideas are material and that “thought is work.” I like his attempts to solidify some of Kant's ideas, but these are so badly worked out the pamphlet is truly mostly a bore and useless.
Lastly, you will not find a mention of "Dialectical Materialism" here. I wish I could say something different, but this book was a mess, and it shows that Marx, Engels, and a few others were really rare in the socialist movement at the time.