I bought this book on dead trees a couple of weeks ago. I've been working through it.

The first third is a typical intro to Scheme - types, code as data, recursion, call/cc, etc., probably more thorough than most intro books, but also faster paced.

The remainder is a much-needed exegesis of R6RS. Every procedure and syntactic form in R6RS is explained, and many dark corners are probed.

There is a chapter of bigger examples at the end. I have not yet read those.

i also have the dead-tree version and am currently on page 100. the book is great, and is perfect for people who already know other programming languages. one think i don't like, though, is that some difficult concepts ad methods ( e.g. call/cc) are not explained as good as other, simpler features. i'm trying to learn scheme because of features like those, and would rather read a few ages more than less.

Scheme was taught at my first university by a "crazy" guy who included Star Wars citations in his lectures, and sometime stood up on his table while imitating samurais. Well he was actually sane, that was just his way to get student's attention. I keep thinking that was a better introduction than any book ;)

scheme is a good language. i've had a nice time with it. that said, i think haskell is also worthy of one's attention. i could elaborate on the reasons why if you wish. a nicely-written guide on the topic is here: http://learnyouahaskell.com/

I went looking in the preface for what's new in the 4th ed. - the only thing I found is on the back cover:

"The fourth edition has been substantially revised and expanded to bring the content up to date with the current Scheme standard, the Revised6 Report on Scheme. All parts of the book were updated and three new chapters were added, covering the language's new library, exception handling, and record-definition features."

"The book offers three chapters of introductory material with numerous examples, eight chapters of reference material, and one chapter of extended examples and additional exercises. All of the examples can be entered directly from the keyboard into an interactive Scheme session. Answers to many of the exercises, a complete formal syntax of Scheme, and a summary of forms and procedures are provided in appendixes."

Dybvig's expository style reminds me very much of K&R, for some reason. Chapters 3 and (in this edition) 12 are probably the most fun to work through to gain an understanding of the language.