The documentation is pretty awful, consisting of a few handbook pages on Drupal.org and exhortations to "figure out what a REST server does and you'll understand how to build one". The combination of Services and REST Server get you part way, but a step-by-step process in how to build a module would be useful.
What version of the module to target? 2.x is fairly stable, with work underway on a 3.x branch.
Which version of Drupal to target? 7.x almost ready, 6.x well-established.
- What is a REST API?
- allows people, including you, to build applications on top of your website's data
- Services Module
- what it does
- how it works
- REST Server module
- GET vs. POST
- Write a custom parser
- Build a resource module to handle GET requests with parameters coming through a URL