[Sincerely Yours ♥]

I think it depends. In one of my English classes, I aced every quiz without ever really reading the book. I just skimmed or looked up summaries online. In the next English class I took, reading mattered a lot more, although I could still pass with looking up summaries online. But it was also a book that could be interpreted differently, so I really wasn't understanding it.

As for other classes, I definitely need to read the textbook to understand the class, such as child development or political science.