What are the main ideas of Marx and Engels's historical materialism theory? What has literature to do with ideology? What is the impact of a cultural-materialist perspective for the scope of what can be considered a legitimate object for literary studies? How does a cultural-materialist perspective change the focus of a critic engaging with a text? What is foregrounded, what is sidelined, and what sort of questions are asked?

Marx and Engels believed that a society organizes itself around the way it produces wealth—in other words, around its economic structure. A culture that derives its wealth from hunting or warfare therefore will valorize the traits, such as physical strength, daring, and courage, that allow it gain wealth from hunting and war. In modern industrial society, Marx and Engels say, we valorize the captains of industry who (supposedly the Marxists say, in actuality, that labor creates all wealth) are responsible for creating wealth through owning the means of production, such as the factories and the banks.
Not only do cultures valorize the people at the top of the economic heap, but societies also create ideologies and mystifications (mental mists, essentially, that fog our understanding of reality) to justify the rich amassing and sitting on such great wealth. This is where literature comes in. The ruling classes are very much going to support works of art and literature, say Marx and Engels, that show the rich in a positive light and the poor as undeserving, or, more precisely, as poor through their own fault. The rich will applaud and economically encourage literature that doesn't raise uncomfortable questions about how they obtain or whether they deserve their wealth. They love literature that promotes, say, Horatio Alger rags to riches myths or that depicts the wealthy as noble, upstanding, and deserving.
A cultural-materialist (Marxist) critic will engage with a text at the level of exposing the ideologies (systems of beliefs) or mystifications it takes for granted. For example, a Marxist might focus, in Dickens's Great Expectations, on Magwitch's contention that some, like him, must live "hard" so that others, like Pip, who he supports as a gentleman, can live soft. To a Marxist, that's a lie. Nobody has to live a hard life so that another can live in splendor. There is enough for all, and it should, such a critic would say, be more equally distributed. He would call Magwitch's statement a lie by which the ruling class has brainwashed the poor into accepting their chains. Another example of Marxist theory would come from John Berger and his exposure of how capitalism uses art in advertising or Raymond Williams's writing on how the wealthy obscure the rural labor that creates their wealth.
It is true that in our society it can been difficult to get a book published that critiques capitalism. For example, Sinclair Lewis had a hard time getting The Jungle published for that reason.