I have a copy of the book checked out from the library. It's a really great intro to writing externals, although a solid understanding of C is really required to understand the book.
The book unfortunately seems to be out of print, but I'd expect that Max/MSP would have some helpful documentation for writing externals. The process in Pd is very similar.
I suppose I could give these source files a look, although I can't say for sure that I'll have the patience to transform them to externals. Uhhh... what do they do? I would guess a name like PVNation would refer to phase vocoder?
Let me know if I can help you with anything!
Brian