"Both classes were given literacy test before and after summer vacation. Sure enough, the students who chose their books did better, improving from the previous summer. Those in the community program showed no improvement"Above is a quote form a study done by Dr. Erin Kelly at the University of Rochester Medical Center. In her work she tried to negate the "summer slide" problem that plagues many teachers.
In her study she allowed small groups of students to pick the type of books that they wanted to read over the summer, and she saw immediate improvement. Give the full article a read here.