I found that any AS and A level book on the subjects will do. I bought a gamsat review guide online which pointed me to the right direction as to where to focus, as well as what to expect at the exam and what strategies to follow to maximise my results. A friend also gave me an mcat guide which had all the material needed, along with little bridges relating it to clinical cases and medicine. I also got the acer books and some other practice papers which were very helpful. I can't go into much detail since I don't want anyone to think I'm advertising anything, so if you want details etc pm me.
One thing you need to keep in mind when you study: Don't try to memorise anything (ok, maybe a few physics formulae) since the information you need will be given to you on the exam paper. Try to understand it as much and as deeply as you can - then go back and revise to remember the stuff - the more you revise, the more you remember. Even though the info will be given to you, if you don't know it and you need to learn it on the exam you won't have enough time to answer all the questions.
Don't know how much help this is... They say as far as chemistry is concerned, you need to be up to 1st year in uni level. Don't let it scare you, just make sure you understand what you study and you'll be fine.
One more thing. Don't overlook the other two sections - you might think you're up to it but they might get you by surprice. I found part I the hardest, even though I don't have a science degree. At least with science the possible answer is only one - you know it, you get it. With poems and stuff you have to guess what the examiner is guessing that the poet is thinking. It can get pretty sneaky.
Good luck with your exams and remember - if others can do it, so can you
