Is there some comprehensive documentation for the SDK somewhere that I've missed?

If you are looking for a help file that explains everything, well, there is no such thing. The existing documentation comes in three parts:

• the SDK readme which explains fundamental design principles,
• the comments in the SDK header files, and
• the tutorials available from the developer forum and my homepage.

Aha. Thanks then. I'll look into them.

Don't forget also the source of some plugins that is available, and can be useful as an example

To add to what bubbleguuum suggested, I'd recommed getting the 0.8 SDK (search the forum). The old SDK included the source to lots of components. These are still good references for general foobar programming even though quite a bit of the API has changed.