How to Design Programs: An Introduction to Programming and Computing, by Matthias Felleisen, et al. The website features the complete text for the September 2003 edition of the book, solutions to exercises, additional problem sets, and errata. The book uses DrScheme, the interactive graphical programming environment for the Scheme programming langauge with Windows, Mac OS X and Unix/X versions free for download. The book and site are affiliated with The TeachScheme! Project, which advocates using Scheme in the introductory classroom. DrScheme is fun to play with, and clearly of educational value. I can imagine volunteering at a local high school to teach with this material.

For a free on-line book on how computer programs work that's a little more college-level, try Structure and Interpretation of Computer Programs by Harold Abelson and Gerald Jay Sussman. With complete video lectures by the authors! This book uses Lisp, not Scheme. The difference between Scheme and Common Lisp, from the Scheme FAQ. (I'd wonder if the origin of the answer softens it in one direction, but the same answer is repeated in the Lisp FAQ.) More such info, suitably wiki-thrashed by the warring factions, at this page on Ward's Wiki.

If you're just in it for the Scheme and don't need to know how computers work (?), see Teach Yourself Scheme in Fixnum Days.

For a more everyday useful implementation of Scheme, try Scheme 48, the Scheme implementation described by The Scheme Underground ("We want to take over the world!") as "an ultra-portable Scheme implementation which is easily interfaced to existing software written in other languages." See also MIT/GNU Scheme.

Once you're an addict: Schemers.org hosts copies of the de facto Scheme standard, R5RS, and more.

I got a bunch of these links from Aaron Hawley's Scheme page, which also nicely links the Free On-Line Dictionary of Computing, great for if you want to know why Scheme is called a functional programming language, why a functional language is a declarative language, why most functional langauges are based on typed lambda calculus, who invented lambda calculus and why he gave up on the project.

Lambda The Ultimate is a programming languages blog worth watching if you're into this sort of thing. A recent entry mentions newLISP, a new Scheme/Lisp fusion that's targetting scripting and web programming as well as artificial intelligence and statistics.

One more thing: Where is all this gorgeous TeX-originated HTML coming from, and why are these Scheme books the first time I've seen it? The answer to the former: TeX2page. To the latter: Because it's written in Scheme.