There may be 10,000 songs out there about love, probably more, but they each are unique. Some love songs have a way of assuaging our grief, bringing us back to the precipice of young love, or reminding us of our grandparents and their timeless bond.
The best marketing tip I recall and utilize every day is the one in which I am reminded to have boldness in the message I alone have for the world. There may be a million writers writing on rest, as in my devotional, A Mom's Battle Cry for Rest. However, when faced with someone who walks up to me and asks about my book or inquires after it online, I can have confidence that I am helping that person. I can have assurance that I'm not selling snake oil or sugar pills.
In the Christian Marketplace, we need to believe not only in our convictions, but in our offering to the world and to God. You have a unique voice and if God has called you to write, then believe that what you are marketing is your best effort and that it will help someone. So whether it's transporting the reader along a fictional journey to love, writing to help someone overcome addiction, or sharing how to find the rest that refreshes, market with the same loving intent of why you wrote in the first place…to glorify God.