The Traveler's Notebook starter kit is an incredibly versatile notebook system from Traveler's Company in Japan. They are the original Travelers Notebook.
The starter kit includes a passport sized leather cover that allows you to customize the notebook to your needs by adding different types of inserts (notebooks and accessories) by way of elastic. You can comfortably fit up to 3 notebooks in 1 leather cover so you don't have to compromise between carrying around a planner, a sketchbook and a notebook. You can have all three in one - or customize it in any way you like. The variety of inserts available allows you to get creative! Zippered cases, pen holders, charms and other types of accessories are also available. You can even use your passport as an insert.