Book help! IMPORTANT!?

hey ive been buying a lot of books to add to my library in my house, and i really do love reading ( hah, you would laugh at this if you knew me...) anyhow, im always wanting to write sidenotes or underline stuff in my books ( i.e.: the odyssey, jane eyre, brave new world,count of monte cristo....) but i hold back from it because i dont want the books to get ruined... do you think its okay to write in the books in pen for future references when i re-read it, or do you think i shouldnt write in it.