It fails as a reference because 1) it is poorly organized: information on each concept (printing, for example), is scattered throughout the book; 2) the index is atrocious. There is no way to find the key description of any part of the language using the index, and I often find important concepts in the book that are not in the index.
The poor index, combined with the scattered organization, means I'm constantly frustrated when I want to look up some particular element of the language.
As an introduction, the book is OK. I would have liked to see some meatier code examples. As another reviewer mentioned, the examples are more about the syntax of the language than actual applications that you might learn from. This is a style point -- if you learn best from large chunks of example code, this is not the book for you.
Also, key modules (sys and math, for example) are barely mentioned.
--Pat / zippy@cs.brandeis.edu