Several months ago, I treated myself to a couple of slim little Japanese notebooks from JetPens. The Monokaki notebook was one of them. It is made by the Masuya company, a manufacturer of writing papers since 1882.
The Monokaki notebook is the perfect size for any pocket at 3 5/16″ x 5 1/2″ x 1/4″. It comes in a clear plastic envelope, with a small translucent bellyband that has some brand info, mostly in Japanese. The cover of the notebook is a light, flexible card stock and has an interesting monochromatic look with just the brand name and a beautiful border design. (It looks abstract at first, but then you realize it shows a fountain pen, ink bottle, pencil, lamp and fishbowl!) A center motif is repeated on the back cover. The exterior card stock is light enough that it would probably be best to use some sort of add-on cover if you’re going to be carrying the Monokaki notebook around a lot.
Monokaki notebook compared to pocket size Moleskine 
Monokaki notebook compared to pocket Moleskine
Inside, you have black endpapers which are partially glued to the cover. There are 8 sewn signatures of 16 pages each, but they are glued tightly into the binding, so it doesn’t open quite flat, and once you’ve pressed it open, the cover doesn’t totally close on its own.
The paper inside has a 7mm grid, with a border all around the edge so you don’t see any lines on the outside edges of the notebook. The paper weight isn’t specified (at least not in English) but it feels quite light and fine and smooth. Heavy wet pens will bleed through, and one of my wider fountain pens bled slightly and feathered, but the other fountain pens I tested worked nicely. There is show-through, as you’d expect with a paper of this weight, and as is common with many other Japanese notebooks such as those from Design.Y and Nolty.
All in all, this is a lovely little notebook if you want a (mostly) fountain pen friendly jotter that is light and pocketable, though the durability of the cover is a concern. I paid $7.75 for it at JetPens, which seemed a bit high for such a small notebook, but when I remind myself that it has 128 pages of good quality paper, it seems more reasonable. An A6 (approximately 4 x 6″) version is also available with ruled or blank pages, priced at $10.