Right now Section I (‘API Design’) and Section II (‘Backwards Compatibility’) are finished. The Sections are lacking readable schemes, I'll draw them later.

Section III ‘API as a Product’ will be discussing non-technical issues

what for the APIs exist;
monetizing APIs;
making sure you understand users' needs and collect proper metrics;
common practices, including AA issues and fraud problems;
organizing docs portal;
open source and community.

I also have more distant plans on adding two more subsections to Section I.

Section Ia ‘JSON HTTP APIs’:
- the REST myth;
- following HTTP spec, including those parts where you should not follow the spec;
- best practices;
Section Ib ‘SDK Design’ covering more tricky issues of having proving UI alongside the API (no specific plan right now)

Translation

I am translating new chapters into English at the moment they're ready. I'm not a native speaker, so feel free to correct my grammar.

Contributing

I am accepting inquiries. Feel free to open issues.

I am NOT accepting pull requests introducing any new content, since I'm willing to be the only author. I would gratefully accept typo fixes, though.

Thanks art.mari.ka for the illustration & inspiration.

Thanks Ilya Subbotin and Fedor Golubev for the valuable feedback.

Thanks @tholman for https://github.com/tholman/github-corners.

Thanks Ira Gorelik for the Aqueduct.

Thanks ParaType for PT Sans and PT Serif.

Thanks Christian Robertson for Roboto Mono.