I'd go with Affiliate links and mark them as such. This is a matter of trust, because there's clearly a conflict of interest. If you recommend something and get

I'd go with Affiliate links and mark them as such. This is a matter of trust, because there's clearly a conflict of interest. If you recommend something and get money for the recommendation, how good are your recommendations after all? But if you have a trustworthy relationship to your readers, it's usually appreciated if they get tips on books or software that helps them. If you get a few bucks for a recommendation, everybody wins. Also, you could start your own product (e. g. an ebook or a tutorial about something), although this requires clearly more effort. The advantage of affiliate is that you get to select the products you recommend, unlike with most ads.