Muji carries some very nice notebooks in its stationery lineup, including this pocket notebook with dark ivory paper and a red cloth page marker. The dark tan paper cover is fairly heavy but the notebook itself is flexible rather than hard. The 5.75 by 3.75 size makes it ideal for pockets or purses, although the 144 pages means it is very thick (about a half inch or more) and therefore could be unwieldy.
The paper is a bit darker than shown in the picture above, and not very fountain pen friendly (some bleed through). Gel inks and ballpoints work well on this very smooth recycled paper so if you just need to jot notes or make some quick sketches this is the right paper for you. Unfortunately the notebook isn't available online, but if you're around a store take a look at this pocket notebook.