Wikisource:DjVu viewer

The Internet Archive BookReader is an open source book viewer developed in HTML/JavaScript by the Internet Archive. It currently serves as front end for millions of books in archive.org, plus other organizations. It has been suggested to support it in either on Wikisource or on Commons to provide a better experience when reading DjVu files, which it might support (not natively according to 2011 docs though).

As of 2016, the Internet Archive no longer has interest in using DjVu files at least for themselves; it's not clear whether other users of the bookreader adapted it for direct usage of DjVu files. As of 2015, there are no plans for PDF.js to support DjVu. It may be easier to convert Magnus' book2scroll to a MediaWiki extension.