I have a growing stack of test swatches that served the purpose of figuring out some technique but don't rise to the level of saleability.
My favorite notebook are the cheap, readily available Composition Books you can get at any stationers, drug store or supermarket. I keep a stack in a drawer and use a different one for every project or topic I'm working on. I've got quite a pile in use. Bullet Journal, Branding, Dye Journal, Website, Product Records, Computer Notes, 5247 Remodel, 3448 Remodel, Photoshop Tutorials, Illustrator Tutorials are the ones currently in rotation. My preferred method of identification has been scraps of duct tape labeled with a series of letters and symbols. It's not particularly attractive and as I start thinking about the new studio I'd prefer something I actually wouldn't mind seeing every day.
So I grabbed a couple of test swatches and stitched them into book covers. Two problems solved.