Anything can be a Notebook
In my twenties, I carried On the Road with me to a lot of places. I read passages from it repeatedly and then wrote my thinking on blank pages and in the margins. At the time, I hadn't realized that On the Road was serving as a mentor text. I didn't even know the term mentor text. I didn't know that my trying to copy Kerouac's rhythms and structures would prove to be a good thing--and that this type of practice would go on to serve me as a writer (and teacher) throughout my life.
Anything can be a notebook. We write our ideas on scraps on thought and only a small percentage of those ideas ever make it to a scrap of paper.