This summer, I'm reading classic Sci-fi, naturally. Like Bester's "The Stars My Destination."
If you're looking for books you haven't read but should, consider the old classics that you can now find on Kindle for dirt-cheap.
Other lists of great works ...
Millenium SF Masterworks
David Pringle's Science Fiction: The 100 Best Novels
Top 100 SciFi Books
NPR's Top 100 Science-Fiction, Fantasy Books
Not all the books on these lists are my favorites, but most are very good. Some are outstanding. There are some noticeable absences -- just try to find James Blish's Cities in Flight near the top of any of these lists. But that's part of the fun -- finding a treasure that others have ignored.