try these they are r really helpful:-
1). INtroduction to Computer Studies by Noel Kalicharan (CXC and Cambridge Editions are both good)
2). GCSE Study Guide Computer Studies by Tony Rackham
3). Information Technology GCSE Workout Series
the 1st one is best for everything for e.g. definitions, explanations and examples
the second and third are also vvvvvvv very goodd!!