Some of the best Smalltalk practitioners have teamed-up to provide a great introduction to Smalltalk and the Squeak environment: Squeak by Example.
The book helps you get started with A Quick Tour of Squeak and guides you through A First Application. The Smalltalk language is introduced in three chapters on Syntax in a Nutshell, Understanding Message Syntax and The Smalltalk Object Model.
Development with Squeak is covered in The Squeak Programming Environment and SUnit. Several of the key classes are presented in chapters on Basic Classes, Collections, Streams and Morphic.
The first edition of the book concludes with chapters on Classes and Metaclasses and Frequently Asked Questions.
The full book, which is about 300 pages, can be downloaded for free. A print-on-demand, softcover version is also available. Stephane Ducasse, one of the authors, told me recently that interest in the book is strong, with more than 11,000 downloads so far.