Scheme is a smaller, more tightly defined dialect of LISP than Common LISP. It is particularly popular for teaching. The best manual for the language is the paper by Clinger and Rees. Despite the intimidating title, it is easy to read, if you are an experienced lisp programmer. Moreover, it is free and concise. The textbook by Dybvig contains more discussion and examples, appropriate for a programmer new to Lisp. It also documents changes and proposed changes since the Clinger and Rees paper.
Three other books may also be useful. The textbook by Abelson and Sussman is intended for beginners with little programming experience. It is a classic, but now starting to look dated. The book by Queinnec discusses details of lisp dialects and their implementations. It is extremely accessible and comprehensive, but intended for mature lisp programmers. The Jones and Lins book is also very accessible.
There is also an official IEEE specification of the Scheme language.
More information about Scheme and Scheme implementations can be found on the Main Scheme page. An ftp repository of Scheme implementations can be found on the Indiana FTP site.
In particular, we have found Scheme48 to be a useful free Scheme implementation. Scheme48 and its design philosophy are described in
Another very nice-looking free Scheme is Elk (Extension Language Kit) . This system uses Scheme as a high-level language to glue together user-defined packages and extensions, e.g. written in C.