Cross-references between books

When my next commit goes through (have to do a new checkout first, I
guess), it will be possible to create clickable cross-references between
books. For example

"... more information can be found in &cite-admin;."

which will resolve to

"... more information can be found in the PostgreSQL Administrator's Guide"

either as a link or in italics, as appropriate. It only gets you to the
table of contents, not to individual sections, but it's a start.

There are also &cite-user;, &cite-programmer;, etc.

--
Peter Eisentraut peter_e@gmx.net