Here is a great resource I wanted to share with you: http://apistylebook.com/
From the website:
The API Stylebook aims to help API Designers to solve API design matters and build their API design guidelines by providing quick and easy access to selected and categorized resources.
In this first version the API Stylebook provides direct links to specific topics within publicly available API Design Guidelines. Instead of reinventing the wheel or searching google for hours, API Designers quickly can find solutions and inspiration from these existing guidelines.
It brings the shared knowledge of a lot of companies together in one location giving you insight in how other (succesfull) companies build their APIs.