Do I have to read a book before I ask a question about it? Can I ask questions about books I haven't finished reading yet? Do I have to read a book several times before I ask a question about it?
There shouldn't be any strict requirements about this.
Some questions, such as reading-order questions, are best asked before someone has read the books. But even for plot explanation or literary analysis questions, it shouldn't be necessary to have read the book before asking questions about it.
If someone who hasn't read the book posts a question to which the answer should be obvious (say, "whom does Elizabeth Bennet marry in the end?" or "who's the main character in 1984?"), then we can mark such questions as bad by downvoting them if necessary. But that doesn't mean we should require people to read the book before posting questions about it. It's perfectly possible to post high-quality questions about something you haven't read. (Source: I do it all the time over on Science Fiction & Fantasy, and have rarely if ever embarrassed myself by doing so.)