Eric Harris & Dylan Klebold
I spent ten years scouring the Columbine coverage, and immersed in it with the other local and national reporters on the story. These are far and away the best profiles/biographies of Eric Harris and Dylan Klebold I encountered in that time. The first is truly a standout, and Lynn Bartels was one of the best reporters on the story, who I actually trusted enough to turn to for help vetting some of the book. Alan Prendergast provides a thorough, textured portrait.
These were published in the first few months after the attack, so they don't include key insights we learned much later from The Basement Tapes, Eric and Dylan's journals, etc. (Those were months and years away from being released.) But they are excellent as backgrounds on Eric and Dylan.
"Fatal Friendship: How two suburban boys traded baseball and bowling for murder and madness," by Lynn Bartels & Carla Crowder, Rocky Mountain News. The Rocky went bankrupt, and dropped all its archives from the web. In 2017, I found this site reprinting Lynn's piece. If that link goes bad again, try googling the title of her piece, and/or her name.
"I'm Full of Hate and I Love It," by Alan Prendergast for Westword.
"A Boy With Many Sides" (on Eric), by By Bill Briggs and Jason Blevins, Denver Post.
Jefferson County Sheriff's Report: Of course this is not a journalistic account, but this section of the official report does a nice, concise job of conveying a lot of crucial information about Dylan Klebold and Eric Harris.