The first thing we would recommend doing is setting up a nice-looking landing page for each of the book projects you're considering. When you create a Leanpub book, you automatically get a landing page where you can add content (like a book cover and a video etc.), and then people can sign up as "Interested Readers." This means that you can gauge interest in each project, and you can interact with "Interested Readers" who opt in to share their email address with you (when you publish the first version of your book, everyone who signed up gets notified, regardless of whether they opted in to share an email address with you).
The next thing we would recommend is that you start publishing content as soon as possible, e.g. once you have two or three chapters written. Interacting with readers is a lot more meaningful if they have already bought your book, and you have already published some content.
Here are some tips for being a successful Leanpub author: https://medium.com/@leanpub/tips-for-self-publishing-and-selling-books-using-leanpub-85a0adea8f1a.