Itoya Profolio Petite Journals
These notebooks are perfect for quick notes. The paper is very good quality and takes fountain pen inks very well. Probably the biggest plus to these little notebooks, for me, is that they are perforated pages and I can tear out the page (even the perforating is of very good quality and the pages come out cleanly and neatly), so that I don't have to keep flipping past old notes to get to current ones, because the old notes can be discarded.