Something short and snappy is recommended for opening a Personal Statement. There are no specific 'rules', though iy does no harm to refer to a book you have read at the very start, and to explain briefly how it astounded or irritated you - in other words, how it made you think in a novel way or caused you to reconsider your existing views. Alternatively, you could make a more general statement, one that highlights your interest. Some examples of recent opening paragraphs are given below:

Studying economics in the sixth form has been fascinating, especially since it complemented many of the political issues I had been interested in, such as the economic problems facing Eastern European states prior to joining the European Union.

During my time doing Economics I have found many aspects of the course hat interest me, such as the debate on whether or not Britain should join the Euro. I believe the decision to commit Britain’s control over interest rates to a European body is dangerous at a time when Britain is relatively thriving when compared with Germany, a country in recession for much longer than the UK.

I wish to be recognised as someone who is open-minded and well educated. That is why I want to achieve a degree in the rigorous, but always changing discipline of Economics.

Whether the economy faces a recession or a boom, it is ultimately there at the heart of the majority of decisions we make in our lifetime; the price we pay for our home, the sum of disposable income we have for life's luxuries and even whether I can afford to pay off my student loan after leaving university.

Economics is like a jigsaw puzzle made of information pieces, pieces shaped by both rationality and randomness, which we put together to create a clearer picture of our world. Economics translates complex real world situations into simple models and its application can solve problems faced by an individual or the global community. It is for all these reasons and more that I love this discipline.

I have always wanted to understand more about the world - not about its physical form or its surface, but about the people in it and the reasons why we have come to live the way we do - the reasons for conflict, government decisions and economic developments.
Referring to Krugman's book would be a very good idea, though don't just state that you have read it. Make sure you select some theme contained within the book, and then devote two or three sentences to explaining why you do or don't agree with it, or how it can be applied in other contexts. The selectors at top universities will certainly expect you to have read a few books outside the syllabus, which can also be in related subjects. Other books you might look at for stimulus include:

Buchholz – New Ideas From Dead Economists (Penguin, 1999)
Levitt & Dubner – Freakonomics; A Rogue Economist explores the Hidden Side of Everything (Allen Lane, 2005)
Ormerod – Why Most Things Fail: Evolution, Extinction and Economics (Faber & Faber, 2005)
Harford – The Undercover Economist (Little & Brown, 2006)
Krugman – The Accidental Theorist and Other Dispatches from the Dismal Science (Penguin, 1999)
Kay – The Truth about Markets (Penguin, 2004)
Ritzer – The McDonaldisation of Society (Sage Publications, 2004)
Joyce – Teach Yourself 101 Key Ideas in Politics (McGraw-Hill, 2001)
Wheatcroft - Yo, Blair! (Politicos Publishing, 2007)
Blackburn – Think: A Compelling Introduction to Philosophy (Oxford Paperbacks, 2002)
Heilbroner – The Worldly Philosophers (6th ed.) (Penguin, 1991)
Ball – Critical Mass: How One Thing Leads to Another (Arrow, 2005)
Fisher – Critical Thinking: An Introduction (CUP, 2001)
Law – The Philosophy Gym: 25 Short Adventures in Thinking (Headline, 2004)
Pirie – How to Win Every Argument: the Use and Abuse of Logic (Continuum, 2006)
Warburton – Thinking from A to Z (Routledge, 2000)
Friedman - The World Is Flat (Penguin Allen Lane, 2006)
Dixit and Nalebuff - Thinking Stategically (Norton, 2005)

Your lack of work experience is no disadvantage, though you ought to try to fix up some short secondment. Where doesn't greatly matter. In your PS simply allude to possibilities you are exploring.

Don't refer to all your subjects, unless you can find specific links with economics, in which case you can talk about contrasting approaches to the same issue. Much better is to place the stress on complementary and transferable skills such as abstract reasoning, robust analysis, logical reasoning, synthesis, exploratory research, measured conclusions, etc. Lots of links between economics and maths work best, so make sure the 'favourite areas' you identify in economics are theoretical ideas and models which involve mathematical techniques. Stress how much you enjoy grappling with difficult problem-solving exercises.

Finally, leave open career possibilities. A one sentence speculative statement (very near the end) mentioning two or three possibles will suffice.

I hope this is helpful. I will be happy to review a first draft of your PS when you have completed it.