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!
I found a used book that was given to someone when he graduated top of his class in the early 1900s. There was an inscription from the principal of his college, signed and with a seal from the college. I stood there gazing at it and couldn't help imagining how he finished reading it, left it here and there in his house and finally, after his time, in the attic of the ancestral home. It was later discovered by his daughter or granddaughter and then when they were cleaning up, sold to a second hand book seller rather than just destroyed.
Contrast that to a PDF which is just as crisp and clean as it was when it was created (and which could, become a format for which we don't have a reader anymore).
MAny a times, the books I read don't have enough place to scribble so I keep a A5 half folded as a bookmark and write notes on that with page numbers. Once the book is over, I read through the points and try to reconstruct.
I use a mechanical pencil myself since I like the teeth and my handwriting is better when I use it. Also, ability to erase (not for cleanliness but more to avoid wastage of space) is useful.