I would say start with your followers first — that’s a healthy list. I think that a lot of discovery happens within the app itself, as well. Other campaigns similar to yours will naturally lead potential backers to your campaign.
Have you listened to Russell and Monica’s podcast? It launched recently and they have a lot of suggestions for getting in front of your ideal audience.
They also have a ton of free resources here: ttp://kickstartyourbooksales.com/free