The book. Details about expanded paperback edition.
There was a handy overview of all the common ideas that got debunked, and I can't find it again, but the major point for me is that the shooters weren't bullied. They were moderately popular, and they were bullies themselves.
The reason I had the url for the article on my clipboard is that I'd been reading discussions of bullying, and word about what the shooters were really like doesn't seem to have gotten out.
It is perhaps unsurprising that the initial reaction when it was believed that the shooters were bullied was for weird kids to get bullied more.
Addeendum: The summary. Link found by andrewducker.