I assume that the original answerer is not affiliated with either work. Is this okay to do to answers? (Or questions too, I guess)
ISBN's are the easiest way to uniquely identify a book.
Instead of a seller site, I'd link to an ISBN lookup, like this. Or maybe link to a google books lookup of the ISBN like this. The beauty of these two is that they in no way support the author or push the book itself. Clicking into them will solely get you more information on the novel (or an avenue to purchase/find more about the author if you so desire).
I think secondary to this would be a third party website like wikipedia. And lastly would be an author or book website like Ælis mentions, if you can confirm that they are legitimate. But like seller sites, these usually have some amount of traffic tracking that users may not want to unknowingly jump into.
Regardless of what you choose, I do think we should try to avoid directly linking to seller sites.
I think it was nice that you added those links, however, I'm not really comfortable with the idea of them leading to Amazon.
In such a situation, I think it would be best if we could:
- Link to the author's website
If not possible:
- Link to a "neutral" website (one that doesn't actually sell the book) like Wikipedia
If still not possible:
- Link to a selling place that proposes the book (like Amazon)