usability.gnome.org is no more.
Well, okay, that’s not quite true The old developer.gnome.org sub-site it redirected to is no more, because all the content has moved, mainly to the Usability Project wiki. Hopefully we’ll get some new redirects in place soon.
EDIT: D’oh, seems the development version was already online too, at http://library.gnome.org/devel/hig-book/nightly. I’ll dump the version from my homepage shortly.