Do be careful when using books that are written for exam boards apart from WJEC. While the theory used in each specification is the same, the structure of each course can be radically different. If you have any questions about this, ask your teacher.
Many of these books should be readily available from larger booksellers. If you are willing to buy secondhand, you can save a lot of money. A certain massive online bookstore often sells beaten Media books for as little as a penny (plus P&P). While it might be great to have a shiny, up to date edition, the theory in a tattered textbook filled with highlighting and notes will be just the same, and for a fraction of the cost!
Finally, check out the LRC, as there are lots of excellent Media resources, including some of these books.
A Level
theory.org.uk - More university-centric, but great fun nonetheless.
Mediaknowall - Clear and concise information on a range of Media theory. An effective online revision guide.
Mediaknowall - Clear and concise information on a range of Media theory. An effective online revision guide.
Abercrombie & Longhurst: Dictionary of Media Studies. (Penguin)
Buckland: Teach Yourself Film Studies. (Teach Yourself)
Branston and Stafford: The Media Student's Book. (Routledge)
Strinati: An Introduction to Studying Popular Culture. (Routledge)
Strinati: An Introduction to Theories of Popular Culture. (Routledge)
AS Level
Bateman et al: AS Media Studies: The Essential Introduction For WJEC. (Routledge)
Bell & Connel: WJEC Media Studies: Study and Revision Guide
A2 Level
Bateman et al: A2 Media Studies: The Essential Introduction For WJEC. (Routledge) - note that because the A2 Media Studies exam is base on 9 texts chosen by the teachers at the schools and colleges themselves, you may find it more useful to buy a more general Media textbook, like The Media Student's Book