The Wikipedia article on Marxist dialectical materialism (always a good place to start!)
The “Di” entry on Marxists.org glossary of terms; “Dialectics” and “Dialectical materialism” are the first terms
The Learn Socialism facebook page, a series of very good and very helpful infographics
What Is Dialectical Materialism?
Kropotkin, The Conquest of Bread
Kropotkin, Mutual Aid: A Factor Of Evolution
Proudhon: What Is Property?
Marxism-Leninism-Maoism basic course
Lenin, The State and Revolution
Trotsky, The ABCs of Materialist Dialectics
can’t be having a dialectics resource dump without anything about dialectical naturalism and an adaptation of leftist theory for the 21st century
Bookchin, The Philosophy of Social Ecology
Cronon, The Trouble with Wilderness