While cleaning up my office to turn it into my workshop, I found an old Day Planner binder (seriously old...talking 2005), and decided to hang onto it. At first I considered deconstructing it and making a custom cover for it, but later realized it would make a great sample book.
What's a sample book?
Simple! Any binder will work for this, but some might work better than others: With a small binder you simply glue a small scrap of your materials (leather, lining fabrics, etc.) onto the pages, and add notes like what type of leather it is (cow, sheep, pig, exotic), how it's tanned (veg tan versus chrome tan), weight/thickness, where you got it, price you paid, how much you have, or any other data you may want to note about it. You can also glue in samples of thread or take notes of edge paint (or proportions of edge paint if you need to mix it to match the color) to use.
Then, when you're designing a project, you can take out the pages from your binder and set them beside each other to determine if you like the combination. Or you may take your binder with you to a leather or fabric store if you need to look for lining material, hardware, or thread to match or coordinate.
Another option is to get a larger binder, and packs of card sleeves or photo pocket pages. Put your sample material on index cards that will fit into your sleeves, and when you want to match your leather to other materials you can just pull out the card and hold it against your options.
I've seen this idea used for cosplay, but has found it's helped organize my leather so I don't have to pull out a whole hide when I'm trying to find a matching thread, or guess if the colors will coordinate when I'm out fabric shopping. Or when I'm feeling a flash of inspiration, I may doodle a design on a blank page, and then jot down what materials I would like to use for it, as I have my samples right there to look at rather than going through all of my materials on hand.