If a book has e. g. 100 chapters, the menu may be a little inconvenient for the user.
and execute the following in the console:
$('div.mw-bookmanagerv2-nav-toc') .find('ol') .append('<li>Extra chapter '+i+'</li>');
you'll have to scroll the page to get to the last chapter, and I don't know if this is the better approach.
One alternative I can think of is to add just a few (10?) chapters to the list and insert an input box after that,
.find('ol') .after('<input type="search" placeholder="Search chapter...">');
which the user could use to type the name of another chapter, and get suggestions as in Vector's search menu, which seems to use this module: