Rite in the Rain Weatherproof Notebook, 4" x 6" Universal Page Pattern Weatherproof Notepad, Brown Cover (No. 446)

Bombproof notebook. Good size for stowing in a back pocket temporarily, though I'd go smaller for a constant pocket carry or even chest rig, where it's just sliiiiightly too large. I've carried one for backcountry SAR for many years, and picked up a few more recently to serve in everyday use, and as a tougher, more compact alternative to a clipboard at work. They hold up great to tough use, especially to sweat, moisture, and friction that would blow through other notebooks. Even if you don't need full waterproofing, but want something that's going to hold up for years, it's a solid option. The double spiral binding is great for usability, and doesn't warp. Lines are well sized. Covers don't crack, in my experience.