It can be, yes.
If you know a book well, then you should still be able to write a good answer even without remembering all the relevant quotes verbatim. That's called an expert answer - you're writing as a person with some level of expertise on the book in question, not just as someone who has an electronic copy with a search facility.
Of course, that doesn't mean all quoteless answers are good. "I remember Character X said something about Y, which probably means the answer to your question is yes" would be a bad answer, but "Character X confirms that Y is true, in Chapter 17 just after Z happens; this means the answer is yes" might be almost as good as providing the actual quote from Chapter 17.