There are several categories of documents listed here:
The comprehensive Scheme FAQ is now a Wiki. Please get involved!
Hari Prashanth is kindly maintaining an archive of videos related to both education and research.
Paul Graham's essays are a trove of additional material about the Lisp family.
These are textbooks dedicated to the introductory programming curriculum and/or beginning Schemers.
These are texts on advanced material that either use Scheme as the object of study or employ Scheme to illustrate key ideas.
The Scheme Repository has a collection of classical papers, as the repository at CMU has an older one.
The archives of the RnRS authors' mailing list, dating back to 1984, have recently become available on-line.
The standards have been moved to their own separate page.