[wheezer256]

Bug

Deleting a single book from the web UI removes that book's value of any single-value text or enumeration custom column from every other book in the library.

In delete_whole_book(), the fallthrough else: branch for non-multiple custom columns does
calibre_db.session.delete(del_cc). For text and enumeration columns, del_cc is not a per-book row: it is the shared value row in custom_column_N, linked to books through books_custom_column_N_link. Deleting it orphans the link for every book that carried that value.

Reproduce

1. Create an enumeration column, e.g. #topic with values Fantasy and Romance.
2. Set #topic = Fantasy on books A, B and C.
3. Delete book A from the web UI.
4. Books B and C now have an empty #topic, and Fantasy is gone from custom_column_N.

We hit this on a real library. Three UI deletes cleared the category from 233 books. An earlier single delete cleared it from about 2000 books.

Fix

Apply the guard the rating branch already uses. A shared (normalized) value row is deleted only when no other book links to it. datetime and comments values are per-book rows, so they are still deleted as before.

if c.datatype in ('int', 'datetime', 'comments') or len(del_cc.books) == 0:
    calibre_db.session.delete(del_cc)

Verified

Ran with the patch applied in a live instance. Deleting one book with #topic = Historical Fiction from the UI removed only that book. The other 49 books kept the value.

(Note: 'int' is in the per-book tuple because db.py names that datatype 'int', while the numeric branch above checks 'integer', so integer columns reach this branch. Those rows have book, not books.)