Until this change is committed, search is simply broken in bs4book.

Take this book as an example (it's not running the latest version of gitbook, but this hasn't changed):
https://bylaws.evangelpresbytery.com

Do a search for "budget". Search will indicate that there is only one place in the book where the word is found: Appendix 1.

In point of fact, the word "budget" is found in several places in Section 4 and Section 6.

Now change ignoreFieldNorm: true as in the patch linked above, and try your search again. Now you get all three locations as search results.

I'm sure this is biting people when they search for stuff in books and don't realize they aren't seeing all the results. But they don't know what they don't know, so people aren't reporting it.