When you are a self-publisher, you are a business owner. You need to choose what kind of business structure you want.
If you do not choose a business structure, then your business is a sole proprietorship by default. Sole proprietorship is the simplest business structure. There is nothing that separates you from your business; this means that your personal finances and your business finances are one. For any other business, this can be a big risk. For self-publishing your book, sole proprietorship is a great business structure to choose because you don't have much to worry about (like in the food industry).
While this is not the "norm" for the publishing industry, it can be an advantage. A duo that splits the tasks by creativity and money, would be ideal.
LLC (Limited Liability Company)
LLC is less costly than a corporation, yet has more protection than a Sole Proprietorship. It differs from a sole proprietorship mainly because you can employ people.