I write on my books. I scribble notes and musings. Not every book gets 'defaced'. Just one particular kind: my recipe books. I write the date I made the recipe for the first time; who I made it for and any tweaks I made to the list of ingredients and instructions. And of course whether it was worth the effort.
I find it's of interest and use to me when I revisit recipes and I also like to think that if my son or daughter ever want to keep certain of my cookery books, they'll have a little history written there of our eating habits. Plus they'll have some handy tips. Even better if they then write about the recipe and start a running historical commentary.
Yes I write on my books. Whisper, whisper I also have overdue library books!