Peter Bartlett wrote:
Maybe the ISBN page could be re-worded to include make the amazon link a referrer link but also include a notice that this is a referrer link and a link to , e.g., [[Wikipedia:Wikipedia's use of referrer links#Why link to Amazon]] which spells out why someone might prefer NOT to click that page, but to go to another bookseller instead.
Also, I have never seen an ISBN reference in Wikipedia unrelated to the actual book title. I'm sure that if someone feels so strongly against Amazon/Wikimedia or their relationship they can still use their minds and skills to make a decision they're happy with regardless of where and how we link (e.g. go to Amazon via direct URL and perform the search manually, search at other booksellers, search on Google, ask friends, place an ad, physically search in bookshops, etc).
The only possible "unethical" possibility would be to actually surcharge the buyer for the reference to Amazon. As long as that's (obviously) not happening, Wikimedia is clean and its spirit is pure--after all, it's offering what most of the commerical community would call a free service: it spares the poor buyer of the agony to manually search the Amazon database (the horror!).
Just my 2c, --Gutza