Theres a book called Columbine by Dave Cullen that came out a few years ago. He also has some articles you can find online about the shooting, all very well researched as far as I can tell.

The narrative right after the shooting was that 2 loner goth kids did a horrible thing to get revenge on their bullies.

However they had a circle of friends, and were more focused on becoming infamous than killing anyone in particular. It wasn't an impulsive attack; they planned for a very long time and wanted to outdo Timothy McVeigh