In another context I have made comments where I'm suggesting that entities be included as well as persons when you're benefiting. I think this would go some way to answering the question about riding associations.
With respect to the books, I picked $30 because it seemed to me one could assume that if something was under $30 it probably wouldn't be seen to have influenced you. But it could be $50, or whatever one thought was sensible. There's a frustration there: I've heard it constantly that people think it's not a problem if it's under the reporting amount, which is $200. I think there could be gifts that are problematic at $100 or $50.