You shouldn't be doing this wider reading with the sole intent of impressing an admissions tutor. You should be doing it due to your genuine interest in the subject area beyond the A-level curriculum. What the admissions tutors want to see is not that you read XYZ book, but how you analysed and reflected upon what you read and how your interests led you to that (and where you are going from there).
It's pretty well known that Oxbridge interviews are not trying to "test" you on what you already know, but to see you apply what you know to an unfamiliar situation. They've stated before that often if an applicant obviously is familiar with the specific details of a particular question they may move on to a new one. The idea being to see how you cope with unfamiliar questions and then how you cope with follow ups to that and adapt your argument in light of new information.
Since law has no subject requirements they aren't going to be expecting any specific background knowledge.