Following PDF is one of the best guides to API design I had encountered online. Document is called: “The Little Manual of API Design” by Jasmin Blanchette from Trolltech, a Nokia company. It includes platform-neutral guidance as to how to create an API for an application or service. You’ll find the very important suggestions and ideas, as long as guidance before starting an actual coding of the API.
Download “The Little Manual of API Design” from this site in DOCX format:
Or in PDF from the following url: