Below you can find scans of Eric Harris' journal entries, as well as some of his writing that has been transcribed for ease of reading. His personal diary is a direct contrast to the things he wrote to appease the adults he'd upset with his behavior. As early as 1996 Eric kept a journal online. This kept up until he had a falling out with Brooks Brown and posted threatening rants about how he wanted Brooks to die. Dylan Klebold gave Brooks the website address in secret and when Brooks saw that Eric had posted the Browns' home address and telephone number, Brooks alerted his parents. After seeing the site for themselves, the Browns contacted the police and Eric's parents as well. Nothing ever came of the police report (which was 'accidentally' lost by Jefferson County for a while following the shootings at Columbine High) but Eric did learn to keep his hostile thoughts to hand-written journals thereafter.
You can see full-sized versions of the scanned pages below in the Columbine Report.
Documents and transcriptions of the content from Eric's websites.
In addition to his online rants, Eric kept hand-written diaries as well. His handwriting was awful so I have a typed transcript of the entries available for those who don't want to decipher his scrawl. I've triple-checked the transcripts for accuracy; any errors that appear are duplications of Eric's errors and spelling habits.