Marking and defacing of books

Silverfish, they are only insects, and fly spray would do the trick - or just squash them (in gloves if you feel better that way).

I've got cluster flies in the roof now that come through the door upstairs. Hate them, but when I was little we used to race cockroaches in the back room. Silverfish at least are quiet and small.

If it's a paperback novel and belongs to me, I'm a spine bending, scribbling, corner-turning horror, and not in the least bit ashamed. It's only a book.

If it's a cookery book, I write improvements or other modifications to the recipe, what works and what doesn't, especially if it's a recipe I use a lot.

If it's a flower or tree or fungus or grass or moss or lichen book, I write where and when I've seen each species. Birds too.

If it's a coffee table book, I don't do any of that. I just look at it.

It can be quite funny, I suppose.
I bought an old western for someone and the main character kept thinning his lips - on each and every page.
The last owner had crossed out the title and written 'The man who thins his lips'.
Fair comment, I felt.