mental images' homepage is http://www.mentalimages.com. For errata and changes that were made after this book went into print, and downloadable .mi scene file examples, see http://www.mentalimages.com/books.html.
The source code for the base and physics shader libraries included with the mental ray distribution is available on ftp://www.mentalimages.com/pub/. See the README file at this location for the exact path.
mental images runs several discussion mailing lists; see http://www.mentalimages.com for details and instructions for subscribing and posting (follow the Forum link).