When I'm coding and have an API question, I have one and only one demand from my documentation source -- speed. I want to be able to quickly find the information I seek. "Now. I want it now."
I use PDF versions of my favorite books and the generated Ruby 2.3.4 API documentation locally, which is all good, but I found something that may be even better. GotAPI has a clean interface, is wicked fast and utterly intuitive. It even holds your recent searches.
I've added it to my browser bar so I can get to it quickly and it's rapidly becoming my reference tool of choice. Highly recommended.