This isn't exactly what we want (since only one index is allowed) but it may give us some ideas (at the very least eliminate tendious hard coding of naviagational links; which would have to be redone whenever a book is reorganized).
As explained at http://www.pmichaud.com/wiki/PmWiki/WikiGroup
They have an index; here for example
http://www.pmichaud.com/wiki/PmWiki/DocumentationIndex
That is where they organize the different "WikiGroups" using a wiki-editable index/TOC just like what Toby and I proposed.
Then, on one of the pages linked from the index you write:
%trail%<<|PmWiki.DocumentationIndex|>>
Which, for the WikiGroup page becomes;
<< WikiStructures | PmWiki.DocumentationIndex | Passwords >>
Now look at the index again;
http://www.pmichaud.com/wiki/PmWiki/DocumentationIndex
Notice WikiGroup is in between WikiStructures and Passwords?
Killer.
-- Daniel Mayer (aka mav)