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.