The Ruy Lopez is one of if not the oldest chess opening and it is certainly one of the most famous and most played. Discovered 'accidentally' by the Spanish Monk Ruy Lopez (and thus also known as 'The Spanish Opening') in the 16th century, the monk made an 'incorrect' third move with white, Bb5 rather than the 'correct' Bc4. While the opening is included in a 1561 chess book, it did not really catch hold until rediscovered by Carl Jaenisch in the mid-19th century whose work is immortalized in part in the Schliemann–Jaenisch Gambit of the opening.
Every great champion has played this opening and it is one of the most popular especially in the open games. More importantly, it is one that even club players can use effectively with a bit of study and this is the book with which to start your study. Shaw does a marvelous job of laying out all you need to know especially from the white side of the board (the Ruy Lopez can be effective from the black side also when pushed that way by a skilled player). But if you're looking for a starter book, this and any of the Everyman Starting Out series that I have studied, which is most of them, are great places to start. Large and easy to read, clear diagrams on high quality no-glare paper makes it easy to study for hours as you work through the various lines.
A must-have for any serious player looking for a serious open game opening.