There are two points weighing against this book as it stands: 1) it's dated; it's based on an older version of Smalltalk/V and uses constructs avoided now (such as the message "become" to change a variable's state). 2) For the person on the fast-track, it's slow. It spends a whole chapter on numbers, and waits three more before discussing classes.
In summary, this is a good book from which to learn Smalltalk and objects. I recommend using Smalltalk Express, a descendant of Smalltalk/V freely available from ObjectShare, along with the book.
The book takes you step by step through problems and takes you to a real discovery because it is honest enough to admit that first attempts turn out to be insufficient in later stages. It, therefore, spares you to wonder how an experienced designer gets everything right the first time, but teaches you to rethink what you have done after each step and how to systematically perform iterative development.
I wholeheartedly recommend the book, which is by the way an excellent fit to the freely available Smalltalk Express distribution (Smalltalk/V).