One of the oddities of the demo version of Aperture 2 is the Help menu is empty. I assume this will be fixed soon and that the shipping version will have the same PDF set Aperture 1 had. (It's another question why Aperture doesn't use Apple's Help system. Sometimes I think it's not really an Apple product at all.)
In the meantime an Apple Discussion Group post pointed me to a site I didn't know about: the Apple Manual page. Every Apple product manual shows up there, sorted by publication date. It's a great resource and I'm going to add it to my custom OS X Search widget.
There's also a "page" (query) for Aperture manuals only, but as of today it only has Aperture 1.x manuals. I assume that's a metadata error and we'll soon see the manuals there.