Marginalia

Readers disagree: is it a “sin” to make notes in a book, or is it a gift to future readers?

From a post on a LibraryThing discussion thread, it’s at least a writing prompt:

I was in the Cincinnati Public Library doing some research when I found [a] genealogy that had writing in the margins of almost half of the pages. At first I was upset, but then it became obvious that the author had come to the library again and again over twenty years to update the book. The most recent entries were in a very frail, shaky script.

What’s the story?