Special:Book doesn't know about books. I know it's absurd, but Wikipedia. https://phabricator.wikimedia.org/T28448
Because $reason, what you ask is actually being done separately, as https://meta.wikimedia.org/wiki/Book_management . Getting an extension enabled for Wikibooks or Wikisource is a practically impossible task. If you could start using the experimental https://www.mediawiki.org/wiki/Extension:BookManagerv2 and report back on your experience with it, I'm sure that the development progress would get much faster.
Nemo