If you don't write in your book for college, either you take outside notes (like in a spiral) or you are wasting your money. I write, highlight, and tab the hell out of my textbooks. I'm very picky though on the way I do it. Yellow highlighters only, thin ones at that, pencil over pen, and small tabs, not the inch wide ones.

As for the library, in college you can't afford to just have the book for two weeks. Even at that, you're better off buying it new instead of handling some disgusting old book that you could get off of Amazon for $5.

As for fiction (the only books I read are classics that are not textbook related), I bookmark and underline or write margin notes in pencil. I usually underline quotes or bracket paragraphs which I find very artistically appealing or beautifully written. Many classics do have those great sentences or "quotables", how I put it. I don't dogear because I buy them new, but at the same time I do like having my thoughts in the novel as I read, as a mental reminder of how I "analyzed" the book.