The American Library Association believes that summer reading programs are important because they encourages reading as a lifelong habit, gets hesitant readers to be drawn in through activities, reading during the summer helps retain skills, and it can create interest in the library as well as books. While the ALA is speaking about summer reading programs, this can also apply to just summer reading in general. You can join a summer reading program or just read on your own depending on what you see fit.
(Pictured is The Alchemist by Paulo Coelho, one of my favorite books)
Any book is fine as long as you are reading something you find interesting and engaging. Find a genre you like, an author you like, or a book that you're curious about and see if you find a book you'd like to read that way. You can also always google bestsellers or popular books if you'd like a variety of recommendations. Hopefully this post motivates you to make a summer reading list or get started on reading those books you've been meaning to.
Link to the ALA summer reading benefits: http://libguides.ala.org/summer-reading/benefits