I uploaded the full text of the book "An Introduction to Scheme and its
Implementation" at https://docs.scheme.org/schintro/. It is linked from
the https://docs.scheme.org/ front page.

The book was written by professor Paul R. Wilson from the University of
Texas, apparently in the mid nineties. It's still a good reference and
has been recommended by e.g. Kawa's author at one time. While there are
now many Scheme books, few of them explain how Scheme is implemented.

I was unable to reach Wilson by email and asked Donovan Kolbly for help.
Kolbly is a principal author of RScheme which was based significantly on
Wilson's ideas. Kolbly was not able to reach Wilson either. We waited 6
months and got no response. Most likely Wilson has retired.

Since the book is available for free on the UTexas website (albeit
buried in an FTP directory) and seems to have been intended as free
course material, I posted it on scheme.org.

Here is the original URL: https://www.cs.utexas.edu/ftp/garbage/cs345/.
That directory contains two versions of the book, schintro-v13 and
schintro-v14. I copied the latter.