What are the rules about linking to external pages, which might be commercial?
For example I referenced The TeXBook and wanted to link to it, so that people who don't now it can find it better. The name is to close to "textbook" which confuses search pages. But what link would be acceptable? Can I link directly to Amazon, which might be understood as advertising (such links could include Amazon partner IDs to receive a commission) or only to Knuths website for it?
The same applies to other books and commercial software related to LaTeX. What would be still acceptable and whats not? Are there pre-defined rules from SE for this.
I thought I just bring this topic up for discussion.