Just in time for the developer workshop at IRCAM, I'm pleased to announce that we've just released the Max 5 SDK for writing external objects in C. What we've put together so far is (almost) a complete rewrite of the documentation I first wrote back in 1992 or so.
This is just the first pass, so expect a fair number of errors and incomplete information. We have much, more to document, but there should be enough text and examples to keep you busy for a while.
Visit the Cycling '74 home page and you'll find a link to the new SDK. One file covers both Mac and Windows development.