Maybe Russell's 'The Problems of Philosophy' if you think you're doing General Philosophy, and Mill's Utilitarianism if you're doing Mill.
I didn't do A-level Politics and I found it quite hard going to start with because I had no detailed knowledge of any political institution in either the UK or the US. So if you haven't done 20th century british history or politics i would recommend skimming through some A-level Politics textbooks. It's taken me a very long time (due to lack of dedicated note-taking this year) to learn what a filibuster is, what a select committee is and does, how much of a majority it takes to pass legislation in Congress etc. And it might just be my tutor, but all of the key books on my reading list for each essay took all this as assumed knowledge. So, I apparently missed the academic debate, did not set up the criteria appropriately and so my essays were average rather than good.
But....enjoy the summer. If you feel a bit guilty for not doing anything, then read something, but there's an awful lot of reading to do once you get here and you might as well make the most of the last free summer ever.