Creating a book section from a book (or a book section at all) leaves out a number of important fields. (chapter/section) title, (chapter/section) author...
You don't need to start with a record for the whole book. If you want Zotero records for academic/technical book chapters (sections), the best way to enter the record is to import the chapter(s) from the publishers' websites. Most publishers will list a book's chapters as a table of contents. Worst case, go to each chapter and import the metadata from that webpage. A few publishers don't list the page range (sometimes available on the website, sometimes not). Some do not include the publisher's city. Other than those omissions complete metadata is available.
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Creating a book section from a book (or a book section at all) leaves out a number of important fields. (chapter/section) title, (chapter/section) author...
Screen shot of a section created from an existing book in 7 beta 74
https://s3.amazonaws.com/zotero.org/images/forums/u4557/rqbxt4i1v498u806cgrv.png
The author (of the chapter) can be entered as a creator the same way this has always worked: https://www.zotero.org/support/kb/edited_volumes_and_book_chapters
Neither of those are things that Zotero could be able to add automatically -- they are by definition not known when creating a section from the book