Primary sources are preferred. URLs are unreliable and transient, but are good sources of ideas, if checked and confirmed, and should be credited along with a primary source. Comments about the story can be used briefly as introduction or for smooth transitions between sections, but a plot summary or book report should not be included. More than half a page of plot summary will be discounted as padding. The textbook and notes are a good starting place, but you will need to go to other sources to write the essay in enough depth.
Web based material is acceptable, with a few caveats.
Be sure that the source is reliable - official NASA sites are good, a web page written by somebody in their basement describing their own particular theory about alien kidnappings isn't. You should not have a reference list consisting exclusively of web pages. However, published journals or magazine articles accessed through the web are referenced in the same fashion as the print version.