J. Rees and W. Clinger (editors), "Revised3 Report on the Algorithmic Language Scheme", ACM SIGPLAN Notices 21, no. 12, 37-79 (December 1986); also an MIT Technical Report.
The R3 report defines Scheme. It's a manual, not a tutorial ΓÇö readable once you have some background in any Lisp, and invaluable for specific details, but likely to intimidate a beginner. I strongly recommend that you get a copy anyway.