Lately I have been including links to Amazon when refering to books and movies I have bought/read/watched. Is it considered bad form to have those links earn me $$ if someone decides to buy the book?

Amazon has its Associates Program. Which lets you have links to amazon products on your site. If someone clicks through to the product, then decides to buy it, you get a small cut of sale. While I don't think I'd go out of my way to list things just on the hope of someone buying something, I do post links to books quite often.

So the question is, is this considered a good or bad thing? Or does it not matter at all?