Best online reference for XSL-FO?

This should be trivial, but I'm having a hard time finding a good, complete XSL-FO reference.  I normally turn to W3 Schools for stuff like this, but their XSL-FO reference
 seems sparse... the examples are shallow and some of the attributes are not described.

Which online resources do you use?  I'm not adverse to buying a book, either, but an online resource is just more convenient.

Thanks,
Colin