The exam is unpredictable and by looking at the 2010 past paper,there have been quite a lot of questions requiring AS knowledge. The Kaplan book covers the whole specification but if you don't have enough time to cover everything then go through the past papers. Don't buy the official book (especially the latest edition) as it only covers the questions that you can find in the past papers. Alternatively, you could buy few books covering all the GCSE science and maths section as well for AS levels (only for science).