I have written in a book. A book I owned not one borrowed from someone else or a library. Also, not a book I was going to trade in at my local secondhand bookstore for more secondhand books. So, I didn't feel it was defacing the book, but it still felt as if I were breaking some kind of cultural barrier.
Quoted from the Amazon book description of "The Reader in the Book: A Study of Spaces and Traces" by Stephen Orgel.
"One of the most commonplace aspects of old books is the fact that people wrote in them, something that, until very recently, has infuriated modern collectors and librarians. ... The underlying question is at what point marginalia, the legible incorporation of the work of reading into the text of the book, became a way of defacing it rather than of increasing its value-why did we want books to lose their history?"
I made notes about what I was thinking as I read the book. I made wonderful notes I wish I could read again now and be inspired by what I thought years ago when I first read the book. But, I gave the book to someone else to read and they didn't value it the way I did. It's gone. I don't even know if they read it cover to cover as I did, or just humoured me when I said how wonderful I thought it was.
Beyond that sad little story, I think people who write in a book are those who did find more inspiration, more to explore, in the book than the people who read it (maybe even loved it too) but did not leave any notes. Leaving a note, making that decision and acting upon it to crack into the pristine pages of a book... it takes guts, for lack of a better word. Writing in a book, leaving a mark is one thing, quite a thing for some. But, that's just the start. Your personal thoughts are there, exposed, for as long as that book is still around.
A book has to be burned to be destroyed. Throwing it into the garbage is not a final end. Someone could still pick it out, clean it up a bit and read it. Composting, is closer to an end, but that takes time. Quite a lot of time and you're not going to be there to guard the spot where its composting for as long as it would take for that book to break down. So, your thoughts will be there beyond the time it takes to jot them down.
Knowing and thinking about all of this, would you write in a book? I still do.