Scheme has an IEEE standard as well as a series of revisions to that standard, the latest of which is named “The Revised(5) Report on the Algorithmic Language Scheme.” The latter is available at the link below. It’s a fairly readable reference and, depending on how well your interpreter’s documentation makes the distinction, you’ll want to refer to it so you’ll know the difference between standard parts of the language and extra features added by your interpreter.
http://www.schemers.org/Documents/Standards/R5RS/HTML/
I’d like to draw your attention to this section in particular, since one of the early exercises concerns numeric representations, and also this next section because the idea of tail recursion comes up early in the text (and as C++ programmers many of us are probably going to be concerned about performance with all the recursion going on):
Besides R5RS there are a set of proposed features for standard Scheme called the SRFIs (Scheme Requests for Implementation). I think of these as similar to the Boost libraries for C++: better to use than a vendor specific feature or library by being defacto standards and because they may one day become part of the actual standard:
A draft of the next version of the Scheme standard is available at http://www.r6rs.org.