I believe that students should have some time off from their studies in the summer, and get a little bit of time to 'recharge.' However, that time of year can also be very helpful for working on improving critical reading skills. Reading is a subject that tends to get pushed to the back of the priority list when studying these days, however it plays a big role - even in things like math and science. The better your reading skills are, the easier it will be for you to understand and interpret what a question is actually asking of you.
So for the summer, what I like to do is get a feel for what time of book(s) my student enjoys and has read in the past. At that point I'll help them pick out a novel to start on as soon as school is out. Unlike a book required for English class though, there are no book reports or tests on it, as I just want the student to read for the sheer pleasure of it. If they happen to finish it before the fall, I'll help them choose another, but if not that ok too. Any reading a student does is fantastic!