The Traveler's Notebook has been my primary notebook for over five years now. Aside from testing things for this guide, it is the only notebook I carry. It was originally released by the Japanese stationary company Midori but proved so popular it has become a separate company. This leather-bound Traveller's Notebook is really just that—a leather binding which you can fill with whatever notebook (or notebooks) you want. For example I use the process described here to keep three notebooks within the covers on my traveler's notebook. I have one dot-grid notebook for planning, one plain notebook as my journal, and one heavier paper notebook for sketching and Copic markers. There are other specialized notebooks available (here's a monthly diary and daily planner), as well as inserts to hold business cards and more. Plenty of third-party sellers on Amazon and Etsy offer all sorts of inserts, which may or may not fit.
As you would expect from a company spun out of Midori, all the paper is fantastic, though not great for fountain pens. I also like that there are replacement bands and closure loops, since closure loops always stretch out. At least with the Traveler's Notebook, you can replace the worn-out loops.
If there's a drawback here, it's the slightly odd size. The full-size Traveler's Notebook is 8.7 by 4.9 inches, which is slightly narrower than A5. It's a little different, but I've grown to love it. There's also a “passport” size at 5.3 by 3.9 inches. Note that there are many imitators. I would avoid the cheap Amazon knockoffs, but there are plenty of people on Etsy and elsewhere making very nice notebooks, often in different sizes. I love my A5 Falcon Traveler's Notebook (€64), which wasn't cheap, took ages to ship (it's handmade), but is a work of art.