Ask HN: What resources are available for learning interesting revenue models?

Many companies (not just tech) provide services to users for free (no monetary cost) while generating revenue in a non-obvious way. For example, recently I learned about interchange with credit card networks, and how a company generates revenue from it.

Are there any resources to learn about interesting or non-obvious revenue models? I realize one could find information on a specific model by researching, but I'm looking for ways to become exposed to the models.