There are many articles and books about good API design. But more often we connect to external systems via API, not developing.
I divide programming interfaces into two big sets – public and not so public. The first category – APIs of big companies like Facebook, Twitter, Paypal, etc. Generally, this kind of API is stable, developer-friendly and well-designed. The second category is APIs of small or middle-sized companies, state services and internal applications in big companies. This article will focus on the second set.