Hey allstar.
I graduated a little while ago from Cambridge and here are a couple of books that were on the introductory reading list back in the day!
Understanding Criminal Law by Clarkson
-a little out of date but is a decent and brief introduction to the theory of criminal law (e.g. what is the purpose of criminal law)
An Introduction to Constitutional Law by Barendt
-a good introduction to the framework in which laws are made. The book goes through what a constitution is and why it matters, before considering the approaches that different countries take.
The Concept of Law by Hart
-pretty heavy stuff. This book is an introduction to jurisprudence or the philosophy of law
Any of the above books is a good start, the third book by Hart is more impressive if mentioned on a personal statement. More importantly, you will need to be prepared to discuss any book mentioned in your personal statement in an interview (if relevant).
Hope that helps!