My current project has me looking into making existing APIs RESTful, and changing to use either JSON or SOAP and WSDL.  I found the following resources to be relevant and/or helpful:

• This Stack Exchange question seemed (to me) to be a good overview of REST as it applies to real world, business APIs.
• A Rest 101 Beginner’s Guide put out by SmartBear (SOAPUI)
• Restful Service Best Practices and this white paper on best practices for building RESTful Web Services by Infosys
• Thoughts on RESTful API design
• Web Services: JSON vs. XML and Stop Comparing JSON and XML
• Understanding the JSON Schema