As a retailer, people who underline and highlight books are one of the many banes of my existence, along with smokers and people who store books below the waterline. One line of highlited text turns a $50 book into a $9.50 'as is' book.
But this savvy reader came up with a terrific solution to the problem of highlighting important text without destroying the physical book- they pasted translucent colored post-it notes over the important passages, yellow, blue and green. Flipping through the book, it looks feathered.
Kudos to you, unknown reader! If I ever have cause to highlight a book, I'm stealing your idea.