You may wish to look at:-
* HCB (as already said) I can’t think of any better examples of classic composition.
* Symmetrical and asymmetrical composition.
* Compositional shapes (s shapes, stars, circles, crosses, triangles, etc.).
* Content and context within the image.
* Emotive issues within the context.
* Golden section/rule of thirds.
* Fibonacci numbers.
* Gestalt theory.
* Always look at your pictures upside down and also consider what they may look like flipped in a horizontal plane (you don’t have to print shiny side up).