The second edition of Friedman, Haynes, and Wand's classic text is the standard introduction to concepts of programming languages. It illustrates the essential ideas of programming languages through a series of interpreters written in Scheme. From the foreword by Hal Abelson: "This book brings you face-to-face with the most fundamental idea in computer programming: The interpreter for a computer language is just another program ... If you don't understand interpreters, you can still write programs; you can even be a competent programmer. But you can't be a master."
If you're using this book, you might find DrScheme helpful, since it comes with an "EOPL" language that includes all the extensions to Scheme and the parser generator, SLLGEN, used in the book.
Website at http://www.cs.indiana.edu/eopl/
Others have found that some material in the first edition is still valuable, and not as well covered in the new version.