Keep in mind that a retailer needs a reason to replace an established card line in their store. They want to make sure every square inch of their establishment makes money and they’re not going to stop selling a profitable line unless they’re absolutely sure the new line (meaning yours) will outsell it. Of course, there’s always the chance they have a line they’re eager to replace but you can’t count on that. Even if you’re planning on licensing the cards – in lieu of manufacturing and distributing them – the company that considers your designs will still be thinking, “Will retailers be willing to get rid of an existing line and take a chance on this one?” continued.. Greeting Card Business Insider Secrets.
These are books I own and recommend.
Some may be out of print, but used copies may be available.
Check link. Start & Run a Greeting Cards Business
Book covers starting a card business in the UK.
Good basic info, but financial/measurement info is in different monetary units.
Creative Illustration Workshop
Exercises for integrating illustration with other mixed-media techniques, and scan and layer your illustrations using a computer.
Hand-Printing Techniques for low budgets. Interesting ways to put imagery on products. Perhaps a good technique for putting our designs on model items to show what they would look like on real products.
Printmaking techniques for fine art and how to transfer images to a variety of surfaces. It includes decals and skins.
Cutting-edge developments in international printmaking. Since printmaking is fundamental in most surface design concepts, this book's goal is a modern approach on a traditional printmaking.