This is a powerful book, and not one that avoids controversy. Let's start with perhaps the most debated premise of this book: "The heart is good". The first reaction of many I know is to quickly exclaim that this is heresy. Well, perhaps not. Lets look at what he means when he says the "heart is good".
First, he is referring to the "redeemed heart" specifically. Using the backing of Scripture (Romans 10:9-10, Ezek 26:36, John 3:7, Gal 6:15, Luke 8:15, Luke 6:44-45, and more) he claims that our hearts are transformed through Christ. This is an entirely Biblical concept.
Second, the heart is not the same as the flesh. Eldridge acknowledges that "part of me doesn't want to love my neighbor..and it is that part I must crucify daily" (page 130) and "Yes, we still have to crucify the flesh on a daily basis" (page 76), and even "I take up my cross and crucify my flesh with all its pride, unbelief, and idolatry" (page 224). Obviously, Eldridge understands and acknowledges that the flesh is sinful, so what does he mean by the "heart is good"? One example is found in Romans where Paul speaks to this very issue: "It is no longer I myself who do it, but sin living within me..For in my inner being I delight in God's law." Romans 7:17-22. (page 76) If it is "no longer I myself" who sin, and my "inner being" delights in God's law, then what exactly is his "inner being", and who is "no longer I myself"? Paul speaks of his redeemed heart and the battle with the flesh. It is critical to note this distinction.
The third element is that our heart reflects God's glory. On page 75 he states that "we were created to reflect God's glory, born to bear his image, and He ransomed us to reflect that glory again". See Romans 8:30, Romans 2:29, 2 Cor 2:4-6, and the discussion in Chapter 4. Nothing in this book would indicate Eldridge is speaking of human glory, or some sort of humanistic agenda as he has been accused. In fact in a prayer he uses (page 176) he states that "I confess here and now that it is all about you God, and not about me", and "I surrender every aspect of my life totally and completely to you" (page 174). He states that "every morning we bring our lives fully back to Christ and under His Lordship." (page 174).
The fourth element of his claim that the "heart is good" is one we have to look at subjectively. Eldridge is speaking of brokenness and its profound impact on our walk with God. The examples of brokenness found in his own life and the lives of others (pages 136, 144, etc) give us some insight into the lie we come to believe: namely that even after redemption we are not capable of being transformed. So deep is this lie that we are bad and unworthy that it keeps us from really living for God. In fact CS Lewis acknowledged (page 212) that "when Jesus told us to love others as we love ourselves it would be a horrible command if the self were simply to be hated."
This is a powerful book, though his writing style may not be for everyone. Eldridge wants to really get close to the heart of the matter, and in doing so uses some often radical concepts as illustrations. However, a deep read here shows a solid Biblical foundation and humility, not humanism.