On the notebook though you'll leave material traces almost naturally. By writing down your thoughts you preserve the process of thinking immediately: every slip, every coffee stain and every torn-out page is a part of this process. Thus you not only add content by writing down your thoughts, but you change the very materiality of your notebook and make it your personal companion. Sure, those flat technical objects are a fetish and show signs of use. Still they are something made to be thrown away.
Books on the other hand are made to be kept. Whereas every crack on a technical object is a tiny disaster, you can actually enjoy the manifestations of your personal history on your notebook.