On pages that define a 'saved wiki book' (see Category:Books), you can generate a pdf by following these steps:
First, you need to load the book.
You may see the 'overwrite/append' dialog if you already have pages loaded, and you can choose an action accordingly.
On the resulting page, you can modify the book if you wish to. Otherwise, just locate the generate 'Download' box, and click 'download'. (You can also choose 'odt' if you want to have an OpenOffice version of the book.)
Your book will then be generated, and you'll find a link on the resulting page to download the book.