"Lies" is about the outright lies, omissions, and boringness of US history textbooks. The book was filled to the brim with interesting historical facts mixed with a dash of outrage at the system.
Loewen loves history. That's why he's sad that history is the most hated subject in school. He blames this, largely, on history textbooks. These textbooks aren't just biased (though they are biased); rather, they take out everything interesting, including all of the controversy, all of the ideas, and anything that makes anyone seem bad.