These texts are sometimes called sourcebooks or readers. The primary documents usually become the groundwork for in-class discussions, hence their importance. Do not take these readings lightly. For example, in the past I have based an entire ninety minute discussion on a primary source as short as one paragraph. To sum up, the only way you are going to get through all the reading is to approach it with the proper attitude, something I have already discussed.
Most instructors assign readings because they want their students to read.
Lectures are one thing. Books are another. And whether your instructor assigns textbooks, monographs, sourcebooks or even novels, the above rules all apply. One last thing. Feel free to assess the assigned readings.