- When you were in high school, the school district owned the books, and marking them was a very serious offense; in college, you own the book, and marking it is a good strategy for learning.
- When you were in high school, you did the reading assignments after the class session; in college, you are expected to do the reading before you show up.
- In high school, relatively few people actually read things because the teacher would tell you everything that the book contained. (That might have been due to the teacher's frustration with students who refused to read anything.) In college, nobody babies you like that. You have to do the reading.
Housekeeping details for textbooks
When you buy your books, keep the receipt. If you bought the wrong book or you have to change your schedule, you can get full credit for an unmarked book within the first couple of weeks.
Books are expensive and some people are dishonest. Don't just leave them laying around on tables. They are very easy to sell, so keep track of your stuff. (This goes for computers and other expensive things too. Not everyone in the college is as sweet and honest as you are.)
Don't obsess about keeping the book in perfect condition. Once you decide that the book is truly yours, go ahead and mark it. Write your name inside the front cover. When buy-back time comes at the end of the semester, you can sell a neatly-marked copy. (And the bad news is that if the textbook comes out in a new edition, you won't be able to sell it at all anyhow.) So highlight important points to help you study.