Hi Losin'. You can write in the books, but you need to do a lot of the exercises in a notebook or on index cards. I feel pretty proprietary about my books, and want to read them when I want to read them, so I'd get my own. YMMV of course.
You should be aware that there is a new Beck book, The Complete Beck Diet for Life that was just released today, and my first flip through makes me think it might even be better than BDS. If you have access to both, I'd suggest flipping through and see which looks like it will better fit your style and goals. There is a lot of commonality between the two books, so you'll get most of the good stuff either way.
Long enough have you dream'd contemptible dreams,
Now I wash the gum from your eyes,
You must habit yourself to the dazzle of the light and of every moment of your life.
-from Song of Myself, Walt Whitman