CURRENT ADVICE
In the book module FAQ - https://docs.moodle.org/24/en/Book_FAQ#How_do_I_search_within_a_book_resource.3F
How do I search within a book resource?
There is no internal search facility, though you can always use your web browser's search capability in print page view.
UPDATE
In Moodle 3.3 I have installed amazon elastic search engine - this is a standard plugin and it will index and search the book module quickly and effectively.