I think that one of the best-kept secrets of Lutheranism are theology books. And if you are reading this, I suspect you read at least your share of those books.
In the last 10 or 15 years, Concordia Publishing House has done a very find job of publishing incisive new books on Lutheran theology and practice. Not to mention the fine older authors whom they've been re-printing. (I'm also a very big fan of Repristination Press, which only does reprints, and has served, I suspect, as a goad to CPH and others for reprinting books which have sadly been allowed to fall out of print).
But I know about these sources. Likely you do, too. But Joe Baptist or Jane Methodist likely don't know about them, and they would profit from them, too. They are unlikely to go directly to Repristination or CPH. Where they will go is to Amazon or other online sellers. And that's where you come in.
I have tried to make a point of writing an online review for every book I read. Whether positive or negative, if I've spent the time reading it, I try to help other readers to know if the book is worthwhile or not. I often use reviews to determine if a book is worth my time. Recommendations from acquaintances whose thinking I trust are better, but failing that, a review is invaluable. And there are excellent books of Lutheran theology which need your review.
Reviews should be thoughtful, insightful, and, when possible, point to the use a reader might find for the book. Here's an example of one I did for Chemnitz's The Two Natures in Christ.
Lots of folks have bad theology. They worship a God whom they don't know well, they pray badly, and they teach others badly. But there are thoughtful readers who can be corrected. A good review may steer them to good theology.