When I worked on the .Net framework at Microsoft we would run usability studies on many of the APIs well before we shipped them. It was always enlightening seeing how professional programmers use your API. We would assume that people would approach the API with a specific mental model for example, only to find that they thought about the API quite differently. I wrote an article in 2004 about how we approached API usability at the time: http://www.drdobbs.com/windows/measuring-api-usability/18440...

The great thing about doing usability studies on APIs is that it is easy to mock up a prototype of the API that people can start to code against. The API doesn't have to work but you get to see how people react to the names of classes, methods, parameters etc. I used to blog about API usability and what we were learning here: https://blogs.msdn.microsoft.com/stevencl/tag/api-usability/

There is a group of researchers and practitioners currently working on API usability. Some good links here: http://www.cs.cmu.edu/~NatProg/apiusability.html