It's mostly a novelty today. Felleisen is a programming language theorist, and him writing a little Java tutorial sounds absurd.

The book itself takes you on a journey to reinvent the Visitor pattern, in a version of Java that predates generics. You can probably find a better explainer for the Visitor pattern in Java.

Which, it seems, is out of print and I guess because a lot of people want a complete set of this series, costs > 500USD to buy used?? Woah. I should get my hands on the others while they're still in print.