Hello Brenan!
Well, I'm an english student and I can suggest you some books that I liked.
My favourite was "The Great Gatsby" of F.S Fitzgerald. I didn't find it very difficult, it has a vocabulary simple to understand. The only problem is that maybe it is a bit long, not very much, but I don't know how long has to be a book for a reading group of summer. Also, with this book, you can do a type of introduction, talking about the "lost generation" and the ideas of that ages. I think the book could interest your students.
I also have another idea. You can choose "The Catcher in the Rye" of J.D.Salinger. I don't know, but maybe this is a very well-known book and they all have read it, but if your students don't know nothing about this book, I think they will like it. This is shorter than the first one, and I think the vocabulary is easily too. I have read this book in spanish so I can't talk about the original one, but I suppose it's easy. With this book, you can do a introduction to the writter, Salinger, because he has a interesting life to study; Salinger life has made Salinger's book. I don't know if you are understanding what I mean, I hope yes.
Well, I have no more ideas now. Maybe I have arrived late and you have started your classes now, but I hope I had helped you a bit.
Bye!