Whenever I write about how important keeping a notebook is to me, people ask me what specific brand of notebook I use. I have no less than 3 notebooks going all the time:
1) an extra-small hardcover notebook that I carry with me whenever I leave the house (which is not often)
3) a fat, paperback-sized, unruled, flexible notebook, which I use at home and in the studio
Of course, only a crazy person juggles 3 notebooks, so just keep things simple and get The Steal Like An Artist Journal instead.