Play the Ruy Lopez by IM Andrew Greet is the book I read when studying the Ruy Lopez. However, it may be a little daunting if you are a begginer due to its massive amount of theory.
Perhaps, something better if you are just starting out on the Ruy Lopez, is Starting out - the Ruy Lopez.
I am a beginner when it comes to openings. I have trialled the Ruy Lopez for a couple of weeks and I have decided that I like it as an opening. I am just wondering what books there are out there that I could buy to improve my knowledge of this opening.
EDIT: I would like a book which covers the main lines and variations for both white and black if it exists