Many of the early Smalltalk books, including the Blue Book which includes a description of Smalltalk written in Smalltalk, and the Green Book with essays on very early Smalltalk history, can be already downloaded from http://stephane.ducasse.free.fr/FreeBooks.html
One thing I've noticed from buying used Smalltalk books is that most of them (apart from the blue/purple/green books) are heavily wedded to defunct implementations. The language is so small it's hard to continue talking about it and not slip into a discussion of the standard library of implementation X.
The best book I've read on OO, by the way, is Andres Valloud's "A Mentoring Course in Smalltalk."
YES! "A Mentoring Course in Smalltalk" is phenomenal! I mean literally it includes a description of implementing perception, you can't get much more phenomenal than that. For real though the part where he discusses conditionals being an indication that you are not demarcating properly at design time was a pivotal moment for me.
