I deal with this issue thus:
1. Import an HTML "text" file that uses all of my standard css rules.
2. Link the file to the css file that is in the book.
3. Add my standard css rules to the css file. (via one snippet)
Note: Each time I run the "Remove unused css rules" tool, my standard css rules are preserved.
4. When finished editing the book I delete the imported HTML file.
5. Run the "Remove unused css rules" tool. Any of my standard css rules that are unused are then deleted.
6. End of story, life is sweet.