Last updated at 6:41 pm UTC on 31 December 2015
Smalltalk 80 : The Language by Adele Goldberg, David Robson; June 1989; Addison-Wesley Pub Co; ISBN: 0201136880
This book is still in print.
The Purple Book is the Blue Book minus "Part Four" (of the Blue Book). Part Four describes the implementation of Smalltalk-80.
If you have Squeak then you have an implementation of Smalltalk-80 which you can read, and in a sense it is a description of itself.
So what you are missing by having the Purple Book plus Squeak, rather than the Blue Book, is only a particular description of Smalltalk-80.