Stephen Wolfram is the founder & CEO of Wolfram Research, creator of Mathematica, Wolfram|Alpha & Wolfram Language, author of A New Kind of Science and other books, and the originator of Wolfram Physics Project.
The part two of this book give a in depth introduction to the internal representation and evaluation of the Mathematica expression, which is normally not touched by other Mathematica book. And is actually extremely useful if you want to learn Mathematica well.
Although this book is based on V5, since the concise of the Mathematica language, it is still of high value to every people who want to learn Mathematica deeply.
Highly recommended to every one who want to learn Mathematica.
Great handbook for someone looking to blend mathematics and computer science. Good balance between instruction and practices/examples. A little outdated, but still teaches the core principles and how to guide oneself through the program.