Can I write in the book?

The other day, someone handed me a book to read. He didn't say a word, tapped me on the shoulder with it and walked away. He's kinda like a mobile library, often lending out his personal books as well as used books, so I fully intend to return it to him. There are markings in the book from the original owner (not him, so it is a used book) and it doesn't feel like it's been read much (paperback). Is it okay for me to make my own marks in pen and/or pencil when I read something significant?