The collection tool, PDF export, and print on demand features are now live on the German Wikibooks edition. PediaPress (who have developed these open source features) are a German company, and want to demonstrate the features at the Frankfurt Book Fair, so it made sense to start in this language. We hope to add the other Wikibooks languages really soon. Next stop: Wikipedia.
We'll also be adding OpenDocument and DocBook export once we've tested them a bit on Wikimedia Labs.
Here's an example full length book rendered with the PDF tool: http://de.wikibooks.org/wiki/Spezial:Sammlung/load_collection/?colltitle=Ben...
(You'll have to click the PDF download button.) As you can see, there are still some hardcoded English texts to get rid of. In terms of output quality, formatting of stuff with underlying HTML in the wiki source texts is the main area of imperfections, since the PDF generator uses wiki-text as a source and gets a bit confused when it encounters HTML. But it should generally ignore what it doesn't understand. If you find cases where it dies, please report them, ideally through the bug tracker at code.pediapress.com (you have to register).
This feature will make it possible to maintain the hierarchical structure of wiki-books through dedicated collection meta-files that are stored in the wiki. The underlying meta-file in the case above is this one:
http://de.wikibooks.org/wiki/Benutzer:Eloquence/Kollektionen/Beispiel-Sammlu...
As you can see, it's a very simple format. These pages can exist either in the user namespace or in the project namespace, and will be automatically detected as "collections" that can then be loaded and exported via the collection toolbox in the sidebar. But for user-friendly PDF download, it's probably easiest to integrate links (in the above format) to ready-made collections into templates, like the existing "printable version" templates.
One of the nicer aspects of this approach is that you can easily have multiple views on the same Wikibook, or create a book pulling from multiple sources. But I also see the collection meta-files potentially useful for other purposes in the future, such as Wikibooks statistics.
When this is available on all projects, I'll write a bit more. If you want to play with an English language version, there's still a demo running at:
http://en.labs.wikimedia.org/wiki/Main_Page
with a full English Wikibooks snapshot database.
Have fun, Erik