Wikipedia makes it very simple to create an eBook from articles on the site:
- Go to the Book creator page to start a book by clicking “Start book creator”
- Visit any pages you want to add to the book and click “Add this page to your book” at the top
- Click “Show book” at the top of the page when you are done
- Select which format (currently PDF, ODT, OpenZIM or EPUB) and then click “Download”
If you’re looking for a way to convert your EPUB Wikipedia eBook to MOBI for a Kindle, check out Calibre.
Update 2015-07-17: Apparently Wikipedia has redesigned how these documents are created and disabled all but PDF export: [Wikitech-ambassadors] Changes to PDF export; ZIM/EPUB will be disabled soon. However, you can still export your book in the PDF format and use Calibre to convert it to MOBI or EPUB.