To understand this book and its theories, one should first understand Jameson's ideas about Marxism. Jameson (along with the majority of Marxists) believes that Hegel's dialectic model was a trustworthy tool for analyzing a culture and its changes. Another important idea to understand is that Jameson borrows from the Vienna School of philosophy, especially Adorno.
From those two ideas, the main argument can be seen clearly. Jameson's Marxist analysis is that the combination of Postmodernism and Capitalism produced a corporate machine to pump out meaningless art en masse. Borrowing from Adorno's Dialectic of Enlightenment, he argues that this constitutes a serious ethical problem, because the people are being systematically removed from meta-narratives and their meaning.
His argument is not that Modernism was correct, nor that Adorno was perfectly correct, but rather, Jameson argues that there is a technical issue when people try to make money through Postmodern art, because it perpetuates the skepticism and rejection of truth-claims.
In other words, the thrust of the book is simply that, although Postmodernism diagnosed many serious problems in Western culture, it is ultimately an incomplete solution, because it systematically removes the culture's ability to discuss ethical truth.
