Well thanks for that. I am saving opf files now

Is there any way to save them in the database so they aren't deleted if you remove the book?
I am trying to convert a lot of books, and/or put metatdata such as series etc. and I get confused when I have too many books displayed at once.
I have the series etc. in the output title, by using an expression rather than typing it in as I used to.
My goal is to be able to easily change all the file names to another format. Currently I have title, author. series, but maybe it should be series, author, title.
Thanks again
Helen