Want to learn more about composition? Ok, then here is a PDF "Artistic Composition for Computer Graphics" where Bruce Gooch and Mario Costa Sousa explain some of the rules of composition. For example:

Artists have their own heuristics for choosing a viewing direction that are reasonably consistent with the psychology results: pick an off-axis view from a natural eye height. Direct 45 degree angles are avoided. Another rule is to have the projections of front/side/top of the object to have relative areas of 4/2/1 on the canvas (often expressed as 55%/30%/15%). The front and side dimensions can be exchanged depending on the object.

This document is full of concise explanations of such rules. You may learn a few (if not many) things about composition.

You have probably read quite a few books on photography in order to learn or polish your skills on composition techniques. And there is always something else to learn. I know that feeling. Though there are things that could be picked up from a sister (or maybe even mother) of Photography - Painting. Many techniques and laws originate from the art of painting. That's why you don't have to forget the resources related to painting.
In article named "Top 10 Tips for Painting Composition" you will find several great tips (which I had no idea of) that could improve your photography skills even further.

Note: The image in this post is a photograph, which I took some time ago and for this post tried to "convert" into a painting.