Heya, I know how much i wanted the answer to questions like this only about a month ago so i am more than happy to help with any- if you want to see my Personal statement or anything like that just PM me (not that it's amazingly good, but i know how badly i wanted to know stuff like that from successful applicants).
Anyway, sorry, in answer to you're actual question I read:
Problems of Philosophy, Russell
Political Philosophy from Plato to Mao, Martin Cohen (i didn't read this in as much detail, and only certian sections- but it is REALLY good for giving a brief overview of the most important Pol Philos works, and it uses alot of extracts from books and accompanies them with a commentry which when you've got something like Leviathon or Wealth of Nations, which are like 500 pages is alot more realistic for a not-yet-undergrad)
The Republic, Plato (this i spent seriously about 75hrs+ analysing, annotating and making notes on. I didn't say a word about it at interview, however as a wise friend of mine pointed out, spending that kinda time analysing a pol and philos book improves you're skills, and makes you think, well, more analytically- so for this it was more the skills than the content- also i probably didn't even scrape the surface of what 3rd years get out of it at oxford. I enjoyed it though, and i think that it did make me take a new perspective on many issues, perhaps most importantly that of the use of argument and analogy)
On Liberty, Mill (No where near- or so i found!- as conceptually difficult as Plato, and he does labour the point a bit, but none the less i found the way one can apply the more ethical side of Mill to situations arising in today's, or conceptual, politics fascinating- and this did come up briefly at interview)
The New Rulers of The World, John Pilger (a journalistic text, getting older now, but i just love Pilger so it was a personal passion really, mentioned in my PS- i think it's nice to have the odd journalist who one attaches to- it shows one must read a fair amount to find these journalists)
I also regularly read the Newstatesman and The Economist, so as to get a healthy balence! (75% of those who apply and read the economist get in for PPE...but then 'there's lies, damn lies and statistics!'... But i like it as reading material personally)
I also meant to read the J-Curve, but only got 1/4 of the way through before interview! You've got plenty of time though, so maybe make room for it- i'm sure knowledge from it would have come in handy.
I'm sorry if this is just a load of rambly uselessness, but i hope it's of some vague help. Finally, i would just say, each of the books above i knew inside out- i had highlighted, annotated, and then written a whole load of notes on them- 30 pgs for the republic!- and that worked better for me than kinda knowing 25 books.