The idea of the plugin is to integrate gtkdoc books into trac's interface. This is accomplished, currently, by and iframe displaying the actual htlm output of gtkdoc, as well as integrating into the search engine in trac.
There's two new user permissions in this plugin, GTKDOC_VIEW and GTKDOC_ADMIN. GTKDOC_VIEW is obviously to view the api and well as search it, where admin allows the addition and removal of books.
As mentioned above the admin interface provides an interface for adding and removing books. Books are keyed on a name given to them at the time they're added, as well as an absolute path on the local filesystem of where the book actually is. This way, you can update the documentation and not have to touch a thing in trac.
At any rate, it's still pretty rough around the edges and lacking a ton of error checking. But feel free to grab it from our extras repo (hg clone http://hg.guifications.org/extras)