Scheme - Which book To Use I am preparing to teach Scheme now, so I need a good reference book. I like books which can dig deep, with none of the beginner level introduction stuff that some books like to spend time on. 

So, what is the best book to use as reference? 

Note : In line with my usual style, I would prefer books that are available in both dead tree and online editions, with the online edition having a higher priority.
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.