None, I believe, except the Bible, has been so universally read and loved by Christians of all tongues and sects (Thomas Carlyle)
The most influential devotional book in Western Christian history (Professor John van Engen)
From the Inside Flap
This book speaks to the soul of every true Christian, always reminding him of the shortness of time and the length of eternity; the fleeting nature of earthly joy as opposed to the eternity of happiness with God. The guide of saints since it first appeared in 1418--it was the sole spiritual reading of St. Therese of Lisieux, who loved it and knew it by heart. Of the universal appeal of The Imitation, the famous theologian Fr. Garrigou-Lagrange noted that "The true mysticism of which it speaks is accessible to all, if they are willing to follow the way of humility, the cross, continual prayer, and docility to the Holy Ghost."