The basic policy is in the [FAQ][1]FAQ:
May I promote products or websites I am affiliated with here?
Be careful, because the community frowns on overt self-promotion and tends to vote it down and flag it as spam. Post good, relevant answers, and if they happen to be about your product or website, so be it. However, you must disclose your affiliation in your answers. Also, if a huge percentage of your posts include a mention of your product or website, you're probably here for the wrong reasons. Our advertising rates are quite reasonable; contact our ad sales team for details. We also offer free vote-based advertising for open source projects.
I'd say it's pretty reasonable, and AFAIK we never had problems with this on TeX.SE.
Edit:
In relation to linking to books and other commercial resources -- I don't mind being referred to Amazon or other authoritative source where said books/manuals/software is available. I won't buy from there in a whim without seeing other options anyway. The only thing I'm against is when the submitter generates profit or other benefit for themselves through linking (affiliation programs, or advertising). Moderate promotion of your own software is fine, especially if it is under open-source license or otherwise freely available with few restrictions. [1]: https://tex.stackexchange.com/faq
