I'm going to review two books today, both of which are seminal and you should definitely read. I don't have time to do a thorough review of either, unfortunately, but if you attend a Christian church and are concerned about the future of Christianity, you should read the one that applies most directly to your church.
J. Gresham Machen wrote Christianity and Liberalism in the 1920's, yet it is still incredibly relevant. If you attend a liberal or mainline church in a denomination where you regularly hear about floor fights over hot-button issues (or perhaps more importantly if you don't hear about them but you know they are happening), you should read this book. It's been a few years since I read it myself so my impressions are a bit faded, but even after the passage of 90 years Machen's challenges to the status quo are striking, his observations prescient. In summary, Christianity and modern theological liberalism are incompatible. Christian churches that have been taken over by liberalism will slowly but surely be stripped of Christianity, and will become not Christ-centered, but man-centered.
Modern Americans are more likely to attend a church of some stripe of evangelicalism, perhaps even a "megachurch". In that case, you really should read Michael Horton's Christless Christianity: The Alternative Gospel of the American Church. Written just a few years ago, it is a damning critique of the "moralistic, therapeutic deism" at the heart of far too many churches. Many Christians react against the liberalism of yesteryear even if they can't fully appreciate its dangers; they vote Republican, attend small group Bible studies, and search for churches with active programs for children. But the Christless Christianity Horton portrays is just as antithetical to true Christianity as the theological liberalism Machen condemns, and it is far closer to home for most of us. Big-name pastors and giant churches offer many different ways to become a better person. Once again, the end result will be a religion that is man-centered rather than Christ-centered.