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Is posting links to commercial content as part of a resource post allowed?

I'm currently working on the Ancient Greek resources page. There are several textbooks I've used and found useful, as well as various other resources that are not open source. What should our policy be for these kind of recommendations; just a name? An Amazon link is okay? Limit to free resources only?

Obviously we don't want to end up with spammy advertising, but sometimes just giving someone a link is what they need.