I would recommend looking into a pop science book rather than a textbook if you'd like to do physics over the summer. Something written for an undergraduate or even the layman would be beneficial. For instance, to prepare for quantum mechanics, one of my professors recommended I purchase The Meaning of Quantum Theory by Jim Baggot ($0.75 on Amazon), it helped a lot conceptually and allowed me to ask questions about the theory without having it heavily obscured by mathematics. That said, I wouldn't argue to read that book when not having been introduced to some modern physics such as the Schrodinger equation in 1 and 3 dimensions in square/cubic wells. QED by Feyman has been interesting so far, but the effort he takes to remove the mathematics makes me slightly less interested; it would probably be interesting to read as a recent high school graduate.
Chemistry is something that I would second, I'm currently studying for the PGRE and having looked at previous tests, the mild amount of more chemical/atomic physics is by far my weakest area; however, when I took chemistry years ago I was not a good student. I am actually ordering some physical chemistry and general chemistry texts now from goodwillbooks.com.
If you'd like math books, there are some paperbacks available about famous math problems. I have a text, Dr. Reimann's Zeroes, about the Reimann Hypothesis as well books on the history of math including Zero, e, and in general. Admittedly, I haven't read them yet, but they're on my to-do list! If you insist on a math book, pick up a cheap linear algebra or discrete math text, it probably doesn't matter too much what as the intro texts (in my opinion) are a dime a dozen these days.
You say you're good with calculus, but there is a lot to study. I'm assuming you took AP calculus, did you take AB or BC? Have you looked into vector calculus yet? Dot and cross products are things you should know, and if you still need to learn them, linear algebra and vector calculus go well together when learning them simultaneously. Also, in my experience, students don't really retain Taylor Series well however, they're very important and you should know them backwards and forwards. Same goes with power series expansions in general, it's important to not let yourself be confused by abstract versus obscurity.
Another option is to find a topic that is related to physics, in fact, this is what I would really suggest. You'll have plenty of time to forward yourself in classes, but your unique interests are what will give you an edge over others for projects as well as serve as a guide for where you want to go in physics. Music and music theory highly interests me, including designing sound studios and electronic instrument effects, amps, etc. I have a book called Designing Hi-fi Furniture which covers some history of furniture as well as designing speaker cabinets. If you share this interest, I highly recommend the text. The paperback is dirt cheap and you can even find a .pdf online for free. PMillett (google it) has several old audio/electrical design texts that are in the public domain as .pdfs.
As far as where to buy them, I never get tired of recommending goodwillbooks.com if you don't object to used books and are in The States. Very cheap texts, very cheap shipping costs. It's perfect if you want to have a couple intro linear algebra or calculus or any other intro subject texts on hand without spending $20 a book. Plus, if you get a dud, a lot of the time you're only out a couple dollars per book, nothing really lost there! Other places include alibris.com and betterworldbooks.com .
