As Bow566 mentioned, a high score on the TSA would be desirable. However, there have been applicants that had a strong TSA score but weren't shortlisted for interview. I, myself, scored badly on the TSA (because of personal circumstances) but was lucky enough to be shortlisted for the interview.
In terms of personal statement, I would recommend focusing on the quality rather than quantity and ensure that there is a link between the super-curricular activities that you choose to do. So for example you could say that you did X which inspired you to do Y and this shows that you're academically inquisitive. But most importantly, when writing it, ask yourself if your personal statement answers why you want to study this subject - it's pointless stating that you've done all these extra activities if you're not telling the admissions tutor why you want to study it.
In terms of books, I would highly recommend Tim Harford's 'Undercover economist'. It brilliantly explains how economics is present within our lives. There are other niche books including 'Freakonomics', etc but I wouldn't recommending reading them as admissions team are likely to see these books stated in many applications - so it wouldn't make you really stand out.
For the management side of the Oxford course, I wouldn't stress too much about it because I think Oxford doesn't expect you to have any prior knowledge on this area as they're likely to cover the basic concepts in 1st year.