Hilariously, as someone with a literal master’s degree in books (master’s in library science, plus a graduate certificate in book arts that ended up only being like 12 credits short of a full MFA ), I used to be the kind of person who couldn’t write in books, then I took a class in grad school that changed me, haha, and now I write in them all the time.
People during the Renaissance period used to write in their books constantly (in fact, if you didn’t, you weren’t considered to be truly “reading”, or at least you weren’t reading studiously), and one of my classes (on Renaissance reading) required me to do so, and I ended up getting so much more out of my grad school reading once I started (plus, class discussions and papers became way easier to write because I could easily flip back through my reading and see the parts that stood out to me), so I just kept doing it after that class.
One Renaissance reader marked up a copy of the First Folio of Shakespeare, and browsing through that copy of it, I found an answer to a question I’d had about a certain passage in one play that I hadn’t been able to find any actual published research on since I first started looking for an answer when I was 16. But thanks to this one reader’s extensive marginalia, that did for me what years of scholarly research could not do. So I’m in favor of writing in books. Future researchers will love you for it.
I also once bought a very cheap used copy of The Odyssey for a class in undergrad (because I’d left my existing nice copy at my parents’ house and suddenly needed it for a class), and the cheapest possible copy I could buy had loads of writing all over it from at least two previous owners. I was annoyed at first and found it distracting, but then I ended up having a great time getting into one-sided arguments with all of their bad opinions in the margins of the book. That was the first time I really wrote in a book I’d owned, since it was already marked up so badly, I didn’t feel any guilt.
So, yeah, as a librarian, writer, and bookmaker myself, I am 100% pro-writing in books. I’d recommend against writing in library books, haha, or I guess anything you want to resell for maximum value, but if you’re unlikely to get rid of your books, you’ll probably appreciate looking back on them someday and seeing your notes, and if you ever do end up getting rid of them, someone else might appreciate your notes more than you think .