After writing a simple Ruby interface into Flickr this weekend, I began to question why Flickr wrote their API the way that they did. Having some level of familiarity with Rails scaffolds and CRUD actions and having somewhat kept up with the state of web services development, I swung by the bookstore and picked up a copy of RESTful Web Services, by Leonard Richardson & Sam Ruby.
This is an exceptional book. I highly recommend this book to anyone who is doing anything technical with the Web.
For a detailed discussion of Representational State Transfer, see Roy Fielding's PhD dissertation, "Architectural Styles and the Design of Network-based Software Architectures". Roy Fielding is co-founder of the Apache project and worked on the original http protocol.