How much additional background reading should I do on my subject

There is no cut and dry answer to this question. The best advice here is to read about what interests you and mention this in your personal statement - this way the interviewer can pick you up on reading that you have actually done.Don't try to become a world expert at age 18 to impress the interviewer because the chances are that any topic that you read up on will have been written on extensively by the interviewer - you may have even already read their books. The interviewer will be much more impressed by your ability to think critically about what you read rather than simply memorising volumes and volumes of information without properly weighing up the content yourself.My number 1 tip with additional reading is to find your area of interest and try and enjoy your reading rather than force yourself to do it (it you are forcing yourself you won't remember any of the information as readily).For example, if you are applying to cambridge for history and really don't enjoy Wilhelmine Germany, don't read about it extensively for your interview just because you feel you 'ought to'. If you eventually have to grit your teeth and cover it in the course of your degree then that is fine, but it will be much harder to 'sell yourself' to the interviewer if you are trying to talk in depth about a topic you don't enjoy.