I just finished this Brennan Manning book today and it was definitely worth the read. I really enjoy Brennan's books but when I'm reading his stuff I always feel like I'm alternating between wading through portions that are a little too intellectual and then soaking in portions where I feel the love of God through his words.
I'd say The Importance of Being Foolish is one of Brennan's most theological-feeling works. He really spends a great deal of time emphasizing the supremacy of Christ, the importance of the cross, and that true life-change only comes through Jesus. The chapter called "Diversions" had me tearing up and was itself worth the price of the book. This is probably my second-favorite Manning book after The Ragamuffin Gospel.
Allow me to ruin the ending for you:
"How long have you been a Christian? How long have you been living in the Spirit? Do you know what it is to love Jesus Christ? Do you know what it is to have your love unsatisfied, endured in loneliness, and ready to burst your restless, ravenous heart? Do you know what it is to have the pain taken away, the hole filled up, to reach out and embrace this sacred Man and say sincerely, "I cannot let you go. In good times and bad, victory and defeat, my life has no meaning without you." If this experience has not illuminated your life with its brilliance, then regardless of age, disposition, or state in life, you do not understand what it means to be a Christian.
This and this alone is authentic Christianity. Not a list of dos an don'ts, not a tedious moralizing, not a list of forbidding commandments, and certainly not the necessary minimum requirement for avoiding the pains of hell. Life in the Spirit is the thrill and the excitement of being loved by and falling in love with Jesus Christ. If the Spirit is not fire, it does not exist. The prayer that rises from my heart is this - if you haven't already, may you come to know in surpassing measure the incredible, passionate joy that I have known in the love of Jesus Christ crucified, the power of God, and the wisdom of God."