Wikipedia books are in HTML format, the only thing that you need is a HTML converter.
I suggest you to use
PrimoPDF, but there is a online tool too, called
Free PDF Converter. If the file converted to PDF, you can convert it(PDF) to the format that you want.
But if you're going to download a book from WikiBooks, then you just need to click on the PDF download link at the right corner of every book:
Or by clicking on the left panel:
Then convert it to any other format
Enjoy!