Have you ever had a reason to read just part of a book? Pages 236-241 of Moby Dick, with no intention to ever read the rest? That seems to be a crucial difference between the two activities.
Moby Dick is probably a poor example. Something like the Iliad or the Bible might be appropriate since extensive studies often focus on a few chapters or verses. Yes, one normally reads the entirety of the work, but studying specific parts seems more apropos.
