Because there is a lot of content of value which isn't in the GitHub project yet.
As I do have a ton of old Smalltalk books (hardcopy, Amazon is your friend) and a hardcopy of all Pharo books (including Pharo for the Enterprise, which is not out yet), I can tell you that, except on the "windows and widgets" front, almost all older books are still full of gems. Smalltalk with Style has been mentioned. Very good indeed.
On thing about Smalltalk is that a lot of it is properly done and has been honed over time.
So, an "old Smalltalk book" is not the typical door stopper kind of book in IT.
(For the story, I've got two Java books which are under my monitors stands, never had any need to look at them since I've put them there. Not so with the Smalltalk books, from which I learn everyday).
For blowing you mind, check Andres Valloud books. There is nothing like it elsewhere methinks.
I tought I understood something about programming. Then I met Smalltalk. And I realized how little I knew. Pharo made me a better programmer. So much that I am now making a living out of Pharo exclusively. This thing makes you realize how much we have been slow on progress, and how current tools, no matter how great, are still stone age style tools.
Being able to extend my own environment, shape it to my needs, well, that's a real "bicycle for the mind". The other tools are really "Anvils for the mind". I've been debugging Javascript in the latest Chrome, hell; the beast can't even locate the line I am stepping over properly (well known problem...).
And I feel happy when doing Smalltalk. I can focus on the real problem. And not battling with all the damn paraphernalia that usually is with the other things (Maven anyone?)
aRant stop.
Phil