One of the foremost sources of information on the negative effects of sugar on our lives is Robert Lustig who is a specialist on pediatric hormone disorders and in the area of childhood obesity at the University of California, San Francisco, School of Medicine.
Here you can see an Authors @ Google talk where he discusses the problems with sugar consumption.
Some folks have taken issue with how far Lustig has taken the negative message on sugar since it seems unlikely that the amounts of sugar found in fruits are harmful to humans apart from other excess sugar intake. This is actually a fairly small point given the high amounts of sugar that most Americans consume.
Another influential author in the area of nutrition and the role of fats and carbohydrates in our diets is Gary Taubes. You can read a piece in the NY Times by Taubes on sugar right here.