Scheme - Which book To Use
How to Design Programs is released under the MIT license, and has the distinct advantage of being integrated directly into DrRacket.  This allows you to work through bite-sized chunks of Scheme, and gives you an easy tutorial-style way to follow the textbook as you explore the language.

Within DrRacket, go to the Language Menu, and the Teaching Languages are in there.  They each reflect portions of How to Design Programs.  The instructions for utilizing the Teaching Languages are contained in the preface to the book.