Please look at the information about preparing for the course, including required reading and set texts, which is on the Faculty website.
Additionally, the following information is from the Director of Studies at Downing College, Dr Marcus Tomalin.
The main areas covered in the first year of the Cambridge Philosophy Tripos are Metaphysics and Philosophy of Mind, Ethics, and Logic. Therefore, it is advisable to acquire a basic knowledge of these. This can best be accomplished by exploring 'classic' texts from previous centuries, as well as more recent discussions. Here are a few specific suggestions:
Nigel Warburton, Philosophy: the Basics (Routledge, 2004).
Adam Morton, Philosophy in Practice (Blackwell, 2003).
Thomas Nagal, What Does it All Mean? (OUP, 2004).
Simon Blackburn, Think (OUP, 2001).
Bertrand Russell, The Problems of Philosophy (OUP, 1912, reprinted 2007).
A. J. Ayer, Language, Truth and Logic (Penguin, 1936).
A. C. Grayling (ed.), Philosophy: A Guide Through the Subject (OUP, 1995).
The volumes in the UCL Press series called Fundamentals of Philosophy are worth reading (especially Ethics by Piers Benn, Philosophy of Mind by Paul Gilbert, Kathleen Lennon, and Stephen Burwood, Philosophy of Science by Alexander Bird, and Philosophy of Language by Alex Miller).
Paul Churchland, Matter and Consciousness (MIT Press, 1988).
Tim Crane, The Mechanical Mind (Penguin, 1995).
Peter Singer, Practical Ethics (CUP, second edition 1993).
J. J. C. Smart and Bernard Williams, Utilitarianism, For and Against (CUP, 1973).
Simon Blackburn, Being Good (Oxford, 2002).
Bernard Williams, Ethics and the Limits of Philosophy (Fontana, 1985).
Jonathan Wolff, An Introduction to Political Philosophy (OUP, revised edition 2006).
Will Kymlicka, Contemporary Political Philosophy (OUP, 2001).
Michael J. Loux, Metaphysics (Routledge, second edition 2002).
R. M. Sainsbury, Paradoxes (CUP, second edition 1995).
Ian Hacking, Why Does Language Matter to Philosophy? (CUP, 1975).
Samuel Guttenplan, The Languages of Logic (Blackwell, second edition, 1997).
A Few `Classic' Texts:
Plato, Meno and Euthyphro.
Berkeley, Three Dialogues.
Hume, Enquiry Concerning the Human Understanding and Dialogues on Natural Religion.
J. S. Mill, Utilitarianism, On Liberty.