As an indie author, simply writing your book is not enough. You need to have a marketing strategy, and let people know about your work. You also need to find your target audience, and then give them a reason to keep buying your work.
One way to do that is to offer extras to your readers. The Write Life calls them “insider treasures.” The idea is to make your readers feel special by giving them a gift:
Many authors create incredible extras inspired by their writing, drawn from their characters or worlds. This can be anything from a map of an alternate world to lovely character cards, or a peek behind the curtain at your creative process.