Goodness, I didn't realise Excel wouldn't carry data validation in cells with sorting. What a pain. There's no easy solution that I can find.

There is a VBA solution but it's clunky. It would involve, before sorting, putting (temporarily or permananently) the list used in data validation in, say, column I of the same row as the data validation in column A (this column could be hidden), then sorting the list including column I, then running down column A re-establishing the data validation from the information in column I (which could include the message '3 authors, 1 book', '5 authors, 1 book', 'multiple authors, 1 book').


I'll write it if it would be useful and a path you're happy to go down.