Writing product reviews are an excellent way to enhance your credibility as an affiliate marketer. This is especially easy with information products like e-books which are relatively inexpensive. After establishing some steady income it may even be worth buying higher-end products, provided you get in touch with the product creator to work out a strategic marketing plan that takes you to the next level.
This has become a popular business model for online entrepreneurs over the past several years, and will probably just continue to grow in popularity. The best thing about selling online courses is that once you do the up-front work in creating the course and setting up your marketing strategy, you can get paid over and over again for work you do once.
If you have pop-ups and optins on every page, I would have to think that it would be a lot harder to convince someone to link to it. Especially since the pages where you have your optins (and probably receive the most traffic) are probably the same pages that you have your best content on as well that you'd be hoping to pitch in your outreach campaign.