Personally I agree with qdtimes2, that it is better off not to
focus only on A level content. The content is pretty outdated, and
quite limited and basic. Content knowledge is also not the main
skill for A level students.
Also, it depends on which subject matter.
If your interest is in Physics, I could recommend you more
things to read up on :)
Personally, even though I specialise in teaching O/A Level
physics, and both Junior and JC Level Singapore Physics Olympiads,
there are still many things about Physics I have yet to read up and
understand.