The RenderMan® Interface Specification, the RISpec, is the official technical specification for the RenderMan Interface. It is quite terse and requires substantial prior knowledge of computer graphics in general and photorealistic image synthesis in particular.

The RISpec 3.2
3.2 PDF Document 
Version 3.2 of the RenderMan Interface Specification is the most recent version of the RI Spec. It supercedes version 3.1, originally published in September, 1989. Version 3.2 contains substantial improvements, updates, and enhancements over its precursor.

The RISpec 3.1
3.1 PDF Document 
3.1 HTML Docs
This document is version 3.1 of the RenderMan Interface Specification of September, 1989. It supercedes version 3.0, originally published in May, 1988. Version 3.1 corrects various typographical and syntactic errors, and a small number of semantic errors present in version 3.0; however, there are no fundamental changes to the structure, concepts or compliance requirements. In addition, version 3.1 introduces a second binding for the RenderMan Interface: the RenderMan Interface Bytestream Protocol (RIB). RIB provides both an archive file format and a network transport protocol for a sequence of RenderMan Interface library calls.

Recommended Reading 
For a more casual reference to the RenderMan Interface, the reader is directed to the new Advanced RenderMan: Creating CGI for Motion Pictures (Tony Apodaca and Larry Gritz 1999), and the classic RenderMan Companion: A Programmer's Guide to Realistic Computer Graphics (Steve Upstill 1989). The first and second printings of The RenderMan Companion (August 1989) correspond (except for minor errata) to version 3.1 of the RenderMan Interface Specification.