If you really want to know more about caffeine from a nutritional perspective, there are few higher rated books on the subject that Caffeine Blues, written by nutritional biochemist Stephen Cherniske.
Cherniske claims that people who consume more than 300 milligrams of caffeine per day are victims of caffeinism: a state of chronic toxicity resulting from excess caffeine consumption and a major contributing factor to heart disease, hypertension, stomach ailments, diabetes, and sleep disorders. Cherniske also warns that most coffee beans are contaminated by pesticides, which harm not only drinkers but also exposed agricultural workers. For conservationists, he highlights the effects of the pesticides on the land and water surrounding the plantations as well as the destruction of the rain forest to make room for coffee plantations. The presence of caffeine in over-the-counter medicines, candy, and soft drinks is stressed, especially in the addiction of children. Cherniske also suggests alternatives to caffeine and ways of quitting the habit.