Along with their Techo and Cousin planners, Hobonichi also manufactures slim A5 and A6 sized notebooks. These notebooks are designed to slip into the back of your planner's protective cover and collect notes and lists that aren't necessarily date dependent.
Like the planner, the notebook is made from 52 gsm Tomoe River paper that is imprinted with a 3.7 mm grid. Since the notebook is designed to slip into the planner's cover, it has a reduced page count, 48 pages (24 sheets), which makes for a very thin (3 mm) book.
The notebooks are sold in a two pack for the crazy cheap price of JP¥540 which equates to US$4.60. $2.30 for a 48 page Tomoe River notebook, I'm in! The only problem is that the notebooks have to be ordered directly from the online Hobonichi store and typically they sell out early in the planner ordering cycle. If you are ordering a planner from Hobonichi it is a great add on item, especially since it typically doesn't add to the cost of shipping.
So how does the notebook perform? Every bit as well as the paper in the planner. Most pens show no bleed through although aggressive markers like the Sharpie best this paper. Similar to other forms of Tomoe River paper, ghosting can be seen through the page but this typically isn't too distracting. Ink shading and sheen show up well on the page but the trade off is that ink dry times are pretty long. The book's stitched binding makes for a book that lays flat and the cover stock is robust and adequate for the job at hand.
If you are interested in an A5 sized Tomoe River notebook but the thought of ordering from Japan seems too daunting, you can order 64 page blank Tomoe River notebooks from Wonder Pens in Toronto for CDN$10.00 which is US$7.40 or Nanimi Paper carries what they call a "Slim A5" which is 20 mm narrower than a true A5. This book has a 3 mm grid, is 80 pages long, and comes in a two pack for US$11.30.