At least she wrote lightly in pencil. At least she didn't editorialize in ink or color passages with highlighter. Often people return library books covered with florescent yellow and green and comments in the margins. Once we charged a man a fee for writing all over a brand-new and expensive coffee-table book about antique silver. While he was paying the fine he said, "I thought you'd appreciate the corrections. I'm an expert, after all!"
Most library-book markers get bored and don't make it past the preface. Some are looking for specific information and so only a chapter or two are marked. Almost always, though, what the marker has marked isn't the important part. She'll pass over "Professor Einstein theorized that space and time are one" and highlight instead "Albert preferred wool socks". Brainiacs are not the ones writing in library books.
So if a book is marked in pencil, I get my Magic Rub (my favorite use of vinyl) and rub out as I read, and I imagine the borrower who made the marks coming to the library for this copy of this book, expecting to find graphite next to her favorite passages, and as I erase, I smile.
A book elaborately coded with multi-colored highlighters, however, we have to throw out.