For several years I've taught an advanced undergraduate programming language course using the second edition of this book. Now I think I see why it's priced at $60, and the third edition is only $44. The third edition loses the simplicity and elegance of the second, replacing it with unnecessary abstraction and complexity (expressed versus denoted values), and treating the fun, hands-on part (implementation in Scheme) almost as an afterthought.
If it ain't broke, don't fix it.