I have to say I loved many of these covers, especially Impossible Motherhood, but in the interest of variety I've decided to make my pick a new one:
The use of white space here is fantastic. I think the designer was brave (and right) to keep the school just on the bottom few inches, rather than front and center on the page. The decision to make the type white, rather than black, was another smart choice. The white, plus the sans serif, all-caps typeface creates a feeling of quiet evenness. The overall feel of the cover is very quiet and peaceful, creating a sharp contrast with the violence that we all know happened. This cover is effective because since we already know what happened at Columbine, we add the foreboding element ourselves. We establish a relationship with the book from the moment we see it, making it that much more compelling to pick up.