The original thesis on REST by Roy Fielding is a good start, to understand the real motivation behind creating it.

Martin Fowler has a good article on the Richardson Maturity Model, and what each part means: http://martinfowler.com/articles/richardsonMaturityModel.htm...

Finally, for an example of why HATEOAS is a good thing, and why APIs should be at level 3 on the RMM, Jon Moore has a good talk on Hypermedia APIs here: http://vimeo.com/20781278

As far as I am aware, there are very few companies actually applying these principles. GitHub is doing something close to it in their API, and I have heard that Google uses this sort of system for their APIs extensively on internal projects. In terms of new-web-API-startup like companies, Twilio/Stripe et. al, I have yet to see many take it this far.