There was a lot of controversy over the journals, notebooks and plans written by Eric Harris and Dylan Klebold regarding the assault on Columbine. A lot of misinformation was circulated and treated as fact before anyone had seen the diary first-hand save the people who found it and the officers who confiscated it. It was nearly impossible to decipher truth from fiction; like many other aspects of this tragedy, figuring everything out would take years.
The shooters' notebooks (often referred to as 'diaries' by many news media outlets) were found only a couple of days after the shootings in thorough searches of the boys' houses and lockers. All manner of outrageous stories aired regarding the findings, though some, like CNN's coverage 5 days after the shootings kept to safer generalities.
Salon.com ran a far more thorough article: "First Glimpse into Eric Harris' Diary", written by Dave Cullen, that came under fire at the time because of the sealed nature of the journals but has since been proven to hold a good deal of information regarding the killers' writings. He has a more in-depth study of what happened at Columbine posted here now.
There was another big deal made elsewhere online about how the boys envisioned killing as many as 500 students, before they would then go on to attack neighboring homes, capping things off by hijacking a jet and crashing it in New York. While the shooters did write about such things, they were in the context of complete fantasy and not something they seriously had plans mapped out to do. More disturbing were the creative writing stories they wrote and the actual plans they'd sketched involving the assault on Columbine.
In the sections below you can find much of the information mentioned above as well as a lot more. There's simply too much to compress into a single page.