Grandparent and parents always taught me books were sacred and shouldn’t be marked or damaged, not gonna side with either side as I do like finding small notes, etc. in 2nd books from time to time. I tackle it with a cheap little Canon Selphy photo printer I use for spitting out random hard copies of photos periodically from my phone. I always make sure to print an extra of any photo I really like. I then use these as my bookmarks in any book I read… I always have 20 to 30 odd non-fictions on the go at once in various states of reading.
This acts as a note keeper, bookmark and sort of time capsule for me. When I finish a book, i’ll always leave it in there. I don’t know why… but I just like the idea of random people (and my kids!) finding these random photos and notes in my books long after i’m gone! The photo in the book also links it to that period of my life.
I found a strange polaroid in a 2nd hand book when I was a little kid, then some years back I used a photo of my own as a bookmark, the memory bubbled back up, and here I am now. Wife and I kept saying we don’t print enough hard copies of photos, so this helps fix that a little bit as well - you could say I now print too many!