Today I got a prototype version of Drupal book.module to produce full DocBook XML format output. I tried it out on a (somewhat out of date) local copy of the Module developer's guide, consisting of 66 sections. Using the stock versions of Norm Walsh and Bob Stayton's DocBook XML to FO DocBook XSL stylesheets, I was able to generate 67 pages of PDF, including images, via Saxon XSL and Apache FOP. There's still quite a bit of work required to get this to produce decent output, but it feels like it's getting close to workable.
The DocBook export code is built around a slightly customized version of Jeff Beal's requires PHP 5, XSLT, and Tidy to be installed and enabled on the server. I've tried to get this working in PHP4, but the XSLT support seems rather flakey.