Illegality is not an absolute value so it had no place in a discussion about ethics. Many laws are ethically wrong.

Then let's just call it "acquiring the book in a way that you don't know if the people involved in creating the book would consent to, but have a lot of evidence to suspect that they actually don't".

From the author's perspective the latter is better (possible word of mouth sales and authoes generally like the idea of people reading and enjoying their writing), so not sure how that works out ethically.

If it's better for them, I suggest asking for permission first, I'm sure they'll be positive to the idea.

How would you prove you weren't going to buy it?

That's the crucial difference between choosing what you think is right and using an external source, you have access to your thoughts while they don't.

A lot of the bands I follow openly support people pirating their music at shows, because those people are ticket and merch buying fans.

Some bands have been scolded and threatened by their publishers for said public voice, and instead choose silence as they could be sued by their producers/publishers.

Creators I have contacted about DRM almost never reply. Likely for similar contractual reasons. They can only legally respond with scolding or silence.

I did have one artist I contacted reply once. They were Christian and all but said I would go to hell for sharing copies of someone elses work.

I asked how they felt about Jesus replicating and freely sharing someone else's bread and fish. They never replied again.

So are public libraries just as "unethical"?

Public libraries typically don’t publish without permission from publishers or authors, there is a difference there.

So libraries are unethical too? Borrowing a paper book from friends?

Libraries get the books from distributors and have licenses for lending them. I’d say that is perfectly ethical.

In my country, you have a legal right to share copies to close friends. I think that’s ethical, but I don’t think it’s ethical when people mass download someone’s work without paying them when they were charging for it.

Piracy vs. not buying it vs. not downloading it are morally equivalent. The actual ethical stance here is whether or not you're paying for DRM; once you've committed to not doing that, whether or not you choose to pirate it at that point is morally neutral because the effect on the author (and publisher et al.) is the same.