During my integration review I noted several things about the new book module which require thought/fixing/clarification.
As none of them were blockers it was integrated however we should still have a look over this list and act upon each point accordingly:

Requiring fixes/changes:

• README.md needs updating to reflect its integration into core. Not urgent but before release probably (defintely "no more developement planned" ).
• The opening line of the boilerplate for each file is inconsistent. Some say part of Moodle, others Book module for Moodle, and others again Book plugin for Moodle.
• $PAGE->set_title calls format_string internally (aweful) but mean no need to wrap things in format_string when setting page title[several files affected].
• $PAGE->set_heading is the same as above.
• booktool_***_extend_settings_navigation no need to check modname !== book that has happened within the calling function book_extend_settings_navigation. (Lots of unused globals in those functions as well).
• book_preload_chapters: next and prev values are never set correctly. Not a bug as they are also not used presently (but should be fixed or removed none the less).
• tool/exportimscp/index.php unused lang string vars.

Requiring clarification:

• I couldn't find or determine a use for book_log function in locallib.php
• PARAM_RAW for the chapter title, is that correct will we allow HTML + friends there?
• Is there a need to add mod_book as a body class. You can be sure the body class path-mod-book has been added for all pages within the book module.
• I couldn't find the use of the chapterscount string. If it is used should that have a {$a}.

Improvements, usability, accessibility, ideas:

• It would be nice if when editing the first chapter (such as when you've just created the activity) if the would freeze the "subchapter" option and add a note about it being frozen because the first page cannot be a subchapter. Just a smidge better than ignoring it for friendliness.
• book_get_toc would be great to convert output to use either a renderer preferably or at least standard output mechanics.
• Simple observation renderers arn't being used, it would be great if we could convert things to make use of a renderer, simple as it may be.