This book unites two approaches to teaching programming languages, one based on a survey of languages and the other on writing definitional interpreters. It has been used as a textbook at over fifteen institutions worldwide, and is referenced by non-academic users on the Web. The book is updated approximately every year. You can learn about the differences between versions, and get the accompanying software, from the book's Web site.

You can get the same PDF version of the book, free of cost, from its Web site. This is to give you a choice: if you want to pay the author, get it from here; if you don't (or can't afford to), get it for free from there.