According to Wikipedia's list of the best selling books of all time, Ellen G. White's book, Steps to Christ has sold 62 million copies and her 5000 articles and 40 published books have been translated into more than 140 languages.
In 1905 White's book, The Ministry of Healing was first published. In the preface she wrote; It is not the creator's purpose that mankind shall be weighed down with a burden of pain, that his activities shall be curtailed by illness, that his strength wane, and his life be cut short by disease. But all too frequently the laws established by God to govern the life are flagrantly transgressed; sin enters the heart, and man loses sight of his dependence upon God, the source of life and health. Then follow the penalties of transgression--pain, sickness, and death.
This book emphasizes the link between diet and spiritual growth. It's a diet book, a self-help book, and a Christian spiritual guide all rolled into one, and it continues to be read widely to this day.