Solr has a HTTP/JSON-based API. However, it isn’t exactly a REST API because it doesn’t really follow any of the rules for a well-defined REST interface. I whine about this sometimes.

The Preface of Developing RESTful Services with JAX-RS 2.0, WebSockets, and JSON is a good place to start if you are thinking of purchasing this book and want to get an overview of what it entails.