The OpenSRS API XML Guide outlines all of the available OpenSRS API commands as well as information on how to integrate with the OpenSRS system. This guide combines all of the information that is the service-specific command reference guides and the API Integration guide.
Attached below, you'll also find separate product guides that contain the commands that are specific to each OpenSRS product. There is one for Domains, one for Trust Service, one for Email, and one for Publishing Service.
Please note that, with the exception of the Email API documentation, the most current documentations are now at domains.opensrs.guide.