Textbooks, always. Personal reading books, never. Not even my name on the first page.
Oh yes. I always thought it was a story until I checked out one of his books - maybe the one where the guy gets hit in the head and changes? After the carnival? I forget. But every single cuss word was edited out. I complained to the library. I rarely even interact with my librarians; they have enough shit to deal with with stupid people, but this really irritated me and I figured they had a record of who all checked it out.
I have also seen library books with:
- anything remotely “sexy” highlighted.
- curse words removed.
- any reference to a disbelief in god removed.
Thankfully not that often. I have borrowed probably thousands of books from one library or another in my lifetime and have only seen it half a dozen times, but that’s enough.
In another thread, long ago, a couple of people did try to make the argument that since libraries are funded by tax money, we all own the books, and thus we all have the right to write in them. And so they would continue writing in theirs.
Many years ago I read The Green Kingdom by Rachel Maddox. It was from the public library and some cretin had gone along blacking out all the swear words and I remember my feelings of indignation and anger. The book contained very little cussing, actually. It was a great book, in its way and to my joy I eventually found a used unmarked paperback that I still read now and again.
I don’t mark books myself, except cookbooks. I regard books as being pretty sacred, which is weird because I’m another one who breaks their spines and folds the pages and reads in the tub and drops salsa on them.
Books are tools, and can be modified to better suit my needs.
I don’t mark up novels, but nonfiction – esp. nonfiction I’ll be referring back to later, for instance as a source for an article – is fair game. When I was writing my dissertation, I’d end up creating my own index on the end-pages of certain books, so I’d have a list of pages that I knew I’d need to go back to (and why).
I think of those books as well-loved. 
I would be very skeptical of someone who claims to really, really love a book when their copy is pristine.
I will write in non-fiction books when I want to (provided, of course, I own the book. I don’t think anyone hear is owning up to writing in library books). I’ve never felt the need to write in a novel. But I find post-its actually more helpful than notes - there’s more room than the margins, they’re easier to find, I can colour-code them.