The Scheme Programming Language – Fourth Edition
The new edition focuses on R6RS, which is the latest version of the Scheme language, and the one that IronScheme supports.
Great reading! 
Thanks to Reddit (and the authors).
Update:
The site seems to have been taken down for unknown reasons. Let’s hope it reappears soon!
Update 2:
The site is now back up
Oops.. Document not found – is link correct?
By: MX on August 3, 2009
at 8:18 am
Seems it was taken down for some reason
Guess it wont be long now.
By: leppie on August 3, 2009
at 5:38 pm