One Principle of Chess Had The Biggest Impact?

I'm rated about 1300-1400 so I"m pretty much a beginner. I'm wondering what book  or books brought you guys from the root to the fruit of chess knowledge? What idea or principle or system of study made the proverbial light go on in your brain to bring you from the depths of obsurity to the heights of prosperity in terms of winning at chess?

Understanding the imbalances -- Jeremy Silman's books such as "Reassess Your Chess".

Studying annotated games.  I recommend Tartakower's "500 Master Games of Chess."  People will tell you not to bother since the book is old and the opening lines have in some cases been refuted, but none of that matters.  In reality you don't need to worry about the quality of the openings in older books. 

What you will learn from that book is strategy and planning.

Another book which I have read recently and highly recommend is Capablanca: Move by Move by IM Cyrus Lakdawala.  That book made more of an impact on my positional understanding than anything else I've ever read.