Enjoy your gap year and don't worry about doing any reading! If you do forget stuff, you will be amazed at how quickly it all comes back to you when you get back to studying. And as for new stuff, its a lot easier when you get to uni and have a better idea of the depth of what you need to know and what you need to know. This will be about the last time ever that you will have a holiday with nothing hanging over your head so forget about it all until September!