One of Mark Guzdial's students created a Squeak API swiki. It could be a good start for a "Squeak in a Nutshell" book. I agree that it would be best to have an online (semi)dynamic reference to reflect the most up-to-date state of the system.
There are other beginnings, such as a number of Morphic references (most of which cover essentially the same topics, though) that could be combined with specific examples, downloadable code, to provide a fairly robust tutoring system. One software company's products were praised for a comprehensive help system with (most importantly) examples.
Finally, there is SqueakDoc, which covers a lot of areas, but may need some editing for consistency (to be considered a "book"). http://minnow.cc.gatech.edu/squeakdoc
So maybe the SqueakDoc should become the starting/continuation point?
Bolot