Tolle's most famous book, "The Power of NOW," was first published in 1997 and has since become a spiritual classic. In this book, he argues that the key to happiness and fulfillment lies in living in the present moment, rather than dwelling on the past or worrying about the future. He also emphasizes the importance of mindfulness, self-awareness, and inner transformation. Tolle's teachings are rooted in his own experiences and insights, as well as his study of various spiritual traditions, including Buddhism, Taoism, and Christianity.