It seems there are several disjunctions between wgContentNamespaces and wgNamespacesToBeSearchedDefault. The latter is what is reflected as "Content" in the Special:Search page (see T170473 for more info).
After consulting with the community (and based on this ticket: T170473#3640902), it's been decided to add the following namespaces to $wgNamespacesToBeSearchedDefault for English Wikibooks:
- Namespaces 102
- Namespaces 110