Book Cover Design and the Problem of Symbolism advises against putting too many elements on your book cover: keep it simple
e-book cover design awards
3 Secrets to e-Book Cover Design Success including keeping your cover simple, make sure it works small, and performs a strategic function.
e-Book Cover Design Awards, September 2011 including four badges awarded for fiction e-book cover designs and one for nonfiction e-book cover design
e-Book Cover Design Awards, September 2011 Part 1