I do not think you are being ‘scrupulous’ or in any way ridiculous, you are simply unsure of the relevant moral principle involved in purchasing products from shops which sell products which cause great harm and unsure of how to apply that principle.
Those seeking to help you should not treat what you have asked as ridiculous and dismiss it but should try to explain to you the moral principle involved and how to apply it.
You want to buy a book to acquire knowledge; This is your intention, and it is good.
The act of buying the book is not wrong, so the act itself is not evil.
So it comes down to the effect of buying the book, whether the good that comes from it outweighs the harm. We should not accept harmful effects if we can reasonably avoid them. Any harm that comes from buying the book is trivial and so you do not need a very serious reason to buy from Amazon rather than somewhere else. That said, if you can without great difficulty or significantly greater expense buy the book from a company which does more good or less harm you should do.