The first diet book in the U.S. was written by Lulu Hunt Peters in 1918, entitled “Diet & Health” A Key to the Calories.” She had quite an insight in spite of the lack of knowledge of her day.
“Lulu Hunt Peters (1873–1930) was an American doctor and author who wrote a featured newspaper column entitled Diet and Health, which she followed up with a best-selling book, Diet & Health: With Key to the Calories. She was the first person to widely popularize the concept of counting calories as a method of weight loss. It was also the first weight-loss book to become a bestseller. Source: Wikipedia
Calories are often misunderstood as to their importance in weight gain or weight loss. The following from Healthline helps to clear up some of the misinformation out there.