I'd like to ask about the meaning of "mark up" in the following context:
Then, on the day we left for our vacation, two books arrived
from Ron Roth. They were heavily marked up, and in a brief note he
suggested I might want to study them. I added the books to our luggage,
and off we went.
Does "heavily marked up" mean that someone made a lot of notes/additions/comments in the text of the book?