I checked out a library book today that's riddled with grad student annotations (the only copy I could find) and I promise you, whatever you write in a library volume will embarrass you in front of that book's future readers—don't do it
I always used notecards as bookmarks for library books and took my notes there. I left one in a volume of HPL's letters and one of my advisors found it when they were doing research months later. Gave it back during one of our book club meetings lol
