Lightwish Swatchbook (they have a portrait version that's a little larger too)!
I tried the Lightwish watercolor swatchbook and talked through the paper feel, weight, and how it performs when you’re swatching watercolor. If you’re building a color library for your paints, this is the kind of tool that can make your setup feel tidy and intentional—and I also mention they have a portrait version that’s a little larger.