"Letters to a Law Student" and "Learning the Law" are good introductory books. From there, you can pick an area or two which you are interested in for further reading. Also if you are thinking Oxbridge, then it's imperative that you keep up with current affairs i.e read the news daily.
Work experience can be useful for career exploration, but is not necessary. I did several placements (from sitting in at high street firms, larger commercial firms to shadowing QCs) but only two sentences of my personal statement mentioned this. Law at university is an academic/ theoretical discipline, and not vocational- as is often mistakenly thought. You do not need to want to be a lawyer to study Law. You just need to be interested in the study of Law. You will need to demonstrate enthusiasm, good analytical and good writing skills.
Other things you could do would be enter essay competitions, debating competitions and summer schools if you are eligible.