Scholastic has a wonderful study done by UT researchers here. In the study, a group of children were given 12 reading books to read during the summer, while the children in the control group were given activity and puzzle books. The results of the study showed that there are enormous benefits to summer reading!
There is also a great New York Times article about the study here.
The decline in reading and spelling skills are greatest among low-income students, who lose the equivalent of about two months of school each summer, according to the National Summer Learning Association, an education advocacy group. And the loss compounds each year.
Barnes and Noble has a great reading program to encourage kids to read. For every 8 books a kid reads, they get a free one. Find more info here!