Fair use is judged on a case by case basis, according to the four factor test.
- Purpose and character of use: Although your use is for-profit, it is transformative. You reproduction of a single quote does not supersede the original book - paying the card game is not something people would do instead of reading the books. When there is a clearly transformative aspect to the work, it can predominate over commercial factors.
- Nature of the copyrighted work: Facts and ideas are not protected by copyright, but their particular expression or fixation is. So if you are using quoting parts of books which have not entered common parlance, because it is a great way of expressing something, you are appropriating the author's particular expression, and as such the author might have a case. However, if you are using the quotes as a commentary on the books (ie. the quote is not meant to express the same idea but "borrow" the book's phrasing, but reference the book) then you would probably fall under fair use. Another possibility is if the phrase has entered common parlance and become an idea unto itself, eg. "you're a wizard, Harry!" is unlikely to get you in hot water (well, with Rowling, who knows - but generally it shouldn't).
- Amount and substantiality: Since you only reproduce one sentence out of thousands, you win on the amount. However, even a small amount may be an issue if it is very substantial. For example, 400 words from an 200.000 word memoir was considered too much - because it was "the heart of the work". So you shouldn't reproduce such passages that are the heart of the book.
- Effect upon work's value: Here you also win for the most part - I doubt that quoting a few words would seriously devalue the book's market. I think arguably, including a major plot twist ("Et tu, Brute?" - if it hadn't been centuries old) could devalue the work and also fall under being "the heart". Another point to consider is that it's not only the impact of your card game, but future implications - what harm could come from letting others reproduce sets of quotes from books? Arguably, none whatsoever, since books of quotations already exist and must have cannibalized all of the business value that you could possibly harm. But nevertheless, that is the standard.
As you can see it's probably fair use, but it depends a lot on specifics and also you can't say for sure how a court would rule. You would be best advised to consult a lawyer. Since you are making a business out of this, you should have a budget for legal matters. Or alternatively, just do your game for books in the public domain. Lots of people read classics.
In addition to the quote, the card would contain interpretive questions. Maybe this is similar to a mini-version of a book club guide you sometimes find as an appendix in books. Would it be fair use to publish guides to conversations about short aspects of published works (not the works as a whole)?
Interpretation is fine. Commentary on books is ubiquitous and old as dirt. Even summaries are mostly fine (just don't actually lift select passages to make your "summary"). See for example SparkNotes publishing plot synopses and such. Obviously, there is a point where you start superseding the original work, but one virtue of a card game is to not have the rulebook be a novel, so this probably is not the case for you.
Regarding your guides on conversation, it's hard to say. If you are quoting whole pages in your rulebook, definitely see a lawyer. If you are just describing in your own words some minor aspect of the plot, that's probably fine.