Chez Scheme is Now Open Source
The Scheme Programming Language
Documentation for Chez Scheme comes in two parts. The The Scheme Programming Language, 4th Edition (TSPL), which is a general introduction to and reference for Scheme. It is available for order direct from MIT Press and various online and local retailers. The full text is available online, along with errata.
The Chez Scheme User's Guide (CSUG) documents features specific to Chez Scheme. A link to the latest version can be found in the Chez Scheme Github project page.