I feel like books are a conversation between author and reader, so I'm happy to conduct that conversation in the margins. When I go back it's interesting to see what I thought the last time I read it - so it becomes a conversation with a past me, too. I love finding marginalia, it brings a books previous lives, well, to life. Of course, I make a distinction between good first editions and reading copies. The FE's get cosseted in archival wrapping and kept out the sun - the reading copies get annotated.
I tend never to write in my books, of any sort (I don't have cookbooks). If I am studying for a class I will, but to me that is not a permanent book, it's a study guide. My books get wear, spine issues and such as I love them and use them, but I don't write in them, nor dog ear them - I want my books to last my lifetime, and more if possible, so I tend to try and treat them gently, but I do not idolize them in untouched form.
In my early days of AD&D, I recall that my friends and I would cross out any unique creatures in the Monster Manual once we had defeated them. We'd work our way through the archdevils and demon lords. Then we got Deities and Demigods and continued the tradition. More recently, I don't usually write in gamebooks, though I often fill them with post-it notes, mostly as bookmarks to different sections, or to note things I want to change.
Though, when I buy RPGs for the middle school that I work at, we scrawl right on the covers with big silver sharpie:
So I have been going through all my old DnD books sharing photos and memories on my HOW I RUN IT instagram account and thought I'd share some examples of my scribbles, notes, and chickenscratch to delight and horrify the folks in this thread. Personally, every time I come across an example of this stuff (most of which is twenty or thirty years old, I absolutely LOVE it).
3e I had tabs some notes in the core books along with and 3-ring binders for extra feats and pdfs of other rules and such. 5e I may update the errata in the core books, but tend to not buy many of the supplements. I tend to run modules from printing them, so I can take notes. Even when I ran the PotA campaign, I made printouts for all the stats and new items I made to go with it, so the need for notes was on a photocopy.
I remember having to initiial the coverletter on military manual updates to acknowledge that the unit copy had had the required pages replaced, or ocasionally, just deleted or inserted.
Yeah, the USAF and US Army occasionally delete whole pages of regulations, and if you're not the admin staff, you might not find out...
Post-it notes leave marks if left in for more than a few years. Early versions were particularly nasty, as they became acidic. Acid+Paper = brown. As in the black ink turns brown, and the paper turns brown. Knock off brands can leave damage after as little as a few weeks.
Oh, and as for laser printed? they can literally pull the toner off.
Work in an Achive for a couple years (I did - US NARA 12NS, Anchorage) and you learn quickly how damaging many convenience products are. The modern steel staple is one of the worst... it can corrode in as little as a year. The Galvanized staples the Army used for side-stapled field manuals, tho', that comes close to indestructible.
Brass staples don't corrode as fast (I've seen 120 year old documents with brass staples - not a wire staple, but a punched brass sheet oval with a spike at one end and hole in the opposite. I've seen these things look brand new and shiny despite the high-acid coarse-pulp paper they were used on going from ecru (presumably) to mil-chocolate in color. And the black iron ink turning to a barely perceptible redbrown.
Brass wire staples of the modern shape are also quite lasting. I've seen 100yo ones that had not discolored at all. I've seena few, however, that leached out copper oxide.