You may have already thought of these but may I give you a few suggestions from my own experience?
Signup for Goodreads and run a free giveaway. This works better than running a free giveaway on Amazon because the people that signup on Goodreads are usually truly interested in the topic and not just downloading free books that they will never read.
Promote on Facebook. Start interesting debates on some of the highpoints in the book. Get people engaged. I recommend adding to your blurb on amazon. There are so many books out there, and sadly, the world prefers the filth, rather than anything pertaining to God. Try quoting from a few areas of your study that raises important and relevant questions, therefore drawing interest of readers to find out more. Add these to your description. Don’t be afraid you will give away too much and thereby be greedy, this will only harm you more than help you.
And lastly, please realize this, you can promote all day long but it is up to God. If He doesn't want it to sale, it won't sale. Sometimes He waits. Sometimes He waits to use a persons work after that person has passed. I know that sounds harsh but it is true. That way God receives all the praise and glory and not the writer. Also consider this: your family could be the true beneficiaries from your studies and relationship with God. Years from now; your grandchildren or great grandchildren may have the opportunity to know you intimately and share, as if they are walking along side you in your walk with God. I had an uncle that I did not get the opportunity to know before he passed, although I feel connected to him. He was a pastor and a writer. Somehow His work was not cherished and no longer exists. (or at least I don't think it made it through the remains of his estate being put on the curb for pickup.) I would love to read His journey: His thoughts. His struggles. His ups. His downs. How God moved Him to abandon his previous job for the ministry.
God prompted me a longtime ago to add all the short stories or studies I write to a scrapbook with details of my journey with Him. Along with photos of life happening around me. One day, those may have impact on my generations and their relationship with God. That is powerful. Don’t ever feel anything pertaining to God is in vain. Don’t base success off of sales. Trust God’s timing.
