Sub-items
It seems like it would be useful to allow for book sections to be attached to (and inherit properties from) books. This would be useful both logically (a section of a book can be attached to the book itself) and practically (instead of manually copying the book's publication information, editors, &c., it could automatically be inherited, saving time and reducing the chance for errors).
That said, using right-click --> Duplicate Item (and then changing the item type), you can pretty much entirely prevent duplicate data entry when creating chapters for a book.
(Started October 23, 2006)
“Books and Book Sections: Avoiding Input of Duplicate Info”
https://forums.zotero.org/discussion/78/
(Started February 5, 2007)
“Hierarchical Item Relationships”
https://forums.zotero.org/discussion/391/2/hierarchical-item-relationships/
(Started June 16, 2010)
“Entire Journals”
https://forums.zotero.org/discussion/13117/entire-journals/
(Started August 24, 2015)
“Hierarchical item types”
https://forums.zotero.org/discussion/51349/hierarchical-item-types/
(Started December 1, 2020 by me)
“Feature Request: Link Book Sections to Book”
https://forums.zotero.org/discussion/86468/feature-request-link-book-sections-to-book/