Some other suggestions:
Stop by the bookstore once a month and read sections of magazine where the editors review readers photographs. They will often make suggestions on cropping or reshooting.
Browse an online gallery like here on Apug or on Ignore the ratings that other people give the photos and ask yourself on each one what works or what can be improved.

Most photography books have a section on composition, but they mainly focus on the rule of thirds and such. The books that i have on composition show examples of many different things like color tone, choice of color/bw, perspecive, etc, but they dont really delve into the why. Perhaps some of the other suggestons people made will help.