Before I read "Common Sense In Chess" by Lasker, I was very poor at Chess. All of my friends were beating me.
After reading the book, I realized that my friends did not understand the basics of opening and development. They were no longer able to beat me. One friends said, "You just got really good, really fast." All I was doing was controling the center, developing, moving no piece twice and maintaining material equality. They were hanging pieces all over the place. I taught it to a friend of mine and he started killing the people he played against at work. He said, "It's like, okay...i'll take this, then that, then this, then that." He suddenly realized how people were always leaving pieces hanging. He never really noticed that before.
The other book that helped me get better was "Bobby Fischer Teaches Chess." It's full of a lot of great mating and attacking ideas.
Which book and why?