When I was a student in the Sign Graphics program at LA Trade Tech, our famed teacher "Doc" Guthrie made us keep what he called a "morgue." It's not as creepy as it sounds -- my morgue is where I keep inspiration for the craft, documenting examples of lettering, sign design, decorative elements, and more. Back in the Trade Tech days, we had to keep our morgues in a three-ring binder, assembling them with magazines and glue sticks. But now, with the recent resurgence in interest in sign painting, there are a ton of amazing practitioners whose work you can see online and from whom you can draw inspiration.
Another amazing resource is Archive.org, a collaboration between a non-profit and organizations like the Smithsonian and the Library of Congress. They're creating a digital library by scanning a ton of old books and other paper ephemera and, lucky for us, there are several amazing sign painting books, some dating back to the 1800s. To see their ever-growing selection of sign writing guides, click here.