Mathematica isn't the kind of program you can intuitively use to any great effect. There's no graphical interface, just a very imposing blank page. Some experimentation with simple commands will tell you how infuriatingly pedantic the input protocols are (type "sin pi" and you will be harranged for not typing "Sin pi"...). Having said that, it IS the largest, most powerful and most versatile program of its kind, so any serious researcher should have it in their repetoire.
This huge book is really a vast lexicon of worked examples on Mathematica's myriad functions. Starting from probably THE most basic foundation (and I kid you not, the first example is 2 + 2 = 4), the book builds up to some really complicated and esoteric stuff. The finicky input structures are all addressed, so no need to worry. If a beginner, one can plod through the exercises in sequence and learn mountains of tricks, or a fluent user could just occasionally reference some abstruse function from time to time. This dual-format is what really makes this book great.
I first borrowed this book from my university library, having been introduced to Mathematica during my course, and it could hold a car up while the spare wheel is changed, such is the magnitude of the program. This book is indispensible, as it is simply not possible to figure Mathematica out properly within a human lifespan.