My name is Sergey, I've been developing APIs for more than a decade (my profile: https://www.linkedin.com/in/twirl/), and I'm writing a book on developing APIs. It is called simply 'The API', and I've just finished writing (then translating) the first section, which is all about the API design. The book is available in English and Russian languages and is being distributed under CC-BY-BC license (e.g. for free).
Complete English text to read in browser: https://twirl.github.io/The-API-Book/docs/API.en.html
You may also find pdf and epub versions at the Github: https://github.com/twirl/The-API-Book