I have spent more time than I would like to admit researching this very subject. Here are a few additional resources you may find useful:
Build APIs You Won’t Hate
https://leanpub.com/build-apis-you-wont-hate
Great book, covers common API related topics. Most examples are in PHP but with some Google Fu you can find relevant Ruby examples.
APIs on Rails
http://apionrails.icalialabs.com/
Another great book on the subject, specific to Rails, but I found it to be too opinionated as I was looking for more of a pros / cons write up of API development. Still worth a read.
JSON-API
http://jsonapi.org
A specification for writing APIs in JSON. Checkout the implementations page for links.
jsonapi-resources seems to be the leading Rails implementation of the specification, although others exist. Has a demo app linked to in the readme.
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I also have the iOS on Rails thoughtbot book and found it to be lacking. It was a great intro to API and iOS development but it glossed over some of the most difficult API topics.
Would I recommend it? Absolutely. Will it be the only resource you need to pickup API skills, no.
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The rest of my API knowledge comes from reading code and Github issues.
Checkout rails-api, keep in mind it will be obsolete come Rails 5.
jsonapi-resources has some good conversations going on.
http://discuss.jsonapi.org/ is a good place to ask some questions. They are trying to get more people involved on their forums.
Versionist is a great gem to read through. Possibly even a great gem to use on your project.
rocket_pants
grape