If I were you, I might think about reading something that I would expect to read in the upcoming school year. I'm assuming you're going to be in 12th grade. In my district, 12th graders focus on British lit. With that in mind, I'd suggest Beowulf, a Shakespeare play like Hamlet or Macbeth, and Heart of Darkness. None of these books are easy, but they are worth the effort. Go to the Enotes study guides for these books and read the summaries and analysis for each (maybe even before you start the book). You'll be surprised how much you learn.
I'd also suggest emailing your possible 12th grade teachers and asking them what you might be reading next year.
You can find lots of book lists online that have good suggestions. Here's one list meant for 12th graders in particular: