REST API design is still a topic under discussion. I'll try to collect list of resources for web developers who want get deeper into it.
Free e-books
Start with Apigee: Web API Design.
Continue with more advanced topics in Thoughts on RESTful API Design .
Addtional interesting apects are covered in HTTP API Design Guide
To get brief understanding on different aspects of API variations refer to Undisturbed REST. Also it covers some instruments for building and prototyping APIs.
Non free books
If you want even more additional info and "other points of view"
- RESTful Web Services, by Leonard Richardson, Sam Ruby (this book is available fo Free on authors site in different e-formats)
- APIs: A Strategy Guide, By Daniel Jacobson, Greg Brail, Dan Woods
Play
- Apiary - powerful service to design, prototype, document and test APIs.
- MuleSoft. Anypoint Platform - Complete platform with API Design Studio for API protyping and documentation.
Specifications / Auto docs
RESTful API concepts are clear, but big and complex app still require more predictable way to organize APIs. People try to find a way to provide standartized way for documenting and organizing REST API protocol.
- Swagger - is a simple yet powerful representation of your RESTful API. Provides tools and standard to create GUI for API docs exploration and testing.
- RAML - RESTful API Modeling Language (RAML) makes it easy to manage the whole API lifecycle from design to sharing.
- JSONAPI - a specification for building apis in JSON. With the aim to make API response data normalized and API self explanatory.
Framework specific
- APIs on Rails, by Abraham Kuri Vargas - Building REST APIs with Rails
