Reference materials, like encyclopedias and dictionaries, are great sources when you need to learn about a new concept or topic. They give an overview of a topic, and usually will point you in the direction of more resources if you want to learn more.
For example, A Dictionary of Critical Theory compiled by Ian Buchanan defines historical materialism as:
A branch of Marxism that—following Karl Marx's own prescription—takes the position that the development of history is not determined by the desires or actions of specific human subjects, but is instead shaped by the objective facts of material existence. History unfolds as the attempt by humans to alter their natural environment to suit their particular needs—this explains, in part, why technology developed at a faster pace in colder climates, the need to defend against the environment was much greater there than in, say, the tropics, where the climate is more congenial. In order to meet their needs humans must work together and produce not only the specific goods they need but societyitself. In doing so, however, divisions between different groups of people whose interests differ arise, the result of which is social antagonism. These groups, which might have been artisans, farmers or merchants to begin with, evolve into classes, of which there are three basic types: landowners, bourgeoisie (i.e. manufacturers and merchants), and workers. The relationship between the classes is dialectical according to Marx inasmuch as their respective interests do not coincide. The working through of this dialectic is referred to as class struggle.
This dictionary provides a concise explanation of the theory of historical materialism. Other reference sources give more information.
The Wikipedia page on Historical materialism outlines the key ideas and implications, and provides a number of references and titles for further reading.
The Encyclopedia of Social Theory provides a different perspective, focusing mainly on the development of the theory and the work of Marx and Engels.
The Concise Oxford Dictionary of Politics by Iain McLean and Alistair McMillan provides another definition that emphasizes the aspects of struggle and contradiction.
These different sources demonstrate how different sources provide slightly different perspectives and interpretations of the theory of historical materialism. All of these are useful sources to help you gain and understanding of the theory.
21st Century Political Science: A Reference Handbook, edited by John T. Ishiyama and Marijke Bruening, includes a chapter on Marxism that outlines the context in which the philosophy emerged, and in which it continued to develop.
The Encyclopedia of Global Studies offers information about Marxism that reflects its global impact and influence, both historically and in contemporary society and politics.
The Encyclopedia of Law & Society: American and Global Perspectives provides an explanation that focuses more on the legal theories related to Marxism.
The Frankfurt School was a group of German American theorists who continued to develop the ideas of Marx and Engels in the 1920s and 1930s. The Encyclopedia of Social Theory offers an excellent article describing the history of the Frankfurt school and development of their ideas.