I _love_ buying a used book and finding a previous reader's notes in the margins. It's the greatest.
A tiny glimpse into a stranger's thinking as they processed the same pages I just did.
Thoughts they may never have shared anywhere else, that may not have made sense to anyone anywhere else. A secret code only for people who just read the last few pages. Shockingly intimate.
I once borrowed a technical book from my school library, and the previous owner who donated that book in the 90s circled the phrase "proof is left to the readers" commonly found grad math book and wrote a big "W.T.F?". It made me laugh so much hha
You might appreciate this one :) https://allpoetry.com/Marginalia
TANGENT One of my all-time favorite websites:
https://www.themarginalian.org/
Highest possible recommendation for its thoughtful, intelligent, entirely human, poetic writing about various interconnected topics. Maria Popova is a treasure.
Nothing quite the same as making scientific conjectures in your marginalia or jotting down ideas in books and then having strangers find them and reinterpret them their own way.
These were fun to find in college bookstores every semester. Not a fan of "highlight it all" but the occasional enlightened soul with a scientific bent.