This is an excellent book, and my understanding of recursion was greatly enhanced. I am going to use Scheme in a programming class next year, and this was just the introduction I needed. But even though this book is thin and is illustrated in a cute way, the last couple chapters of this book are very difficult. The day I finished reading it, I went to the doctor and my blood pressure was up a good deal, because of the brain-wracking I undertook while reading it (mainly the ninth, next to last chapter). It's a classic, and you don't actually need a Scheme interpreter to use it. I didn't use one, though I made sure to do the exercises on paper, and checked them carefully. If you get this book, prepare for quite a (brutal) mind stretcher.