Just thought of something else: check your book settings to see how many entries it supports.

I would think it would have prevented you from adding entries past the limit, but it may not work as I expect - I've never tested it.

EDIT: check here for restrictions on number of entries: https://www.writing.com/main/books/action/view/entry_id/698878

According to this, the "novella" type only accepts 25 maximum.

But the size is shown to be 30 entries. Maybe you can change it to "Collection" or one of the others without altering anything else. /EDIT