For you guys wanting to learn so IA over the summer, just be clear that there is no need to do this. I have never met a single person who did this. Just what are you expecting to get from Cambridge if youre learning the content yourself without the lectures and supervisions?
Ive heard of people who read ahead on 1 or 2 courses, but even that was rare. If you do do this please dont spend money on any textbooks. Lecture notes are available for free online and are perfect for the course. They strip away everything thats not needed and teach what you do need in a concise way. You wont be wasting any time trying to cross reference between the syllabus and your textbook.
Finally, I recommend Groups over any course to read ahead on. Generally found to be one of the more difficult courses, but its a very interesting subject and you wont have seen anything like it before in A-level.
You've just finished STEP, concentrate on your upcoming exams then take at least a month off (preferrably do nothing til September). Youll be doing plenty of maths next year so why start early?