The inside cover of the book, which was recovered from the teenager's home in October 2017, reads: "Sorry if this is found I have committed one of the worst atrocities in British history or I killed myself."
As well as professing his "love" of murderers in the book, the boy also drew a picture of what prosecutors described as the "perfect school shooter".
In one passage, the boy claimed: "Everyone is filthy and deserve to be shot, including me. I'll play the role of god and decide who a let live and die."
Elaborating on that alleged intention, he wrote in the diary: "I have a plan, a great f****** plan."
Referring to carrying out a shooting with his then-girlfriend, he said: "I'll make some explosives then we'll find a way back to Northallerton and we'll begin our assault on that f****** school."
Another page is devoted to "stuff we need" in order to execute a school shooting, including napalm, firearms and pipe bombs.
In other entries, the boy described the human race as a "vile species which needs to die out" and humanity itself as "a curse and a burden".
One extract reads: "We need to aid natural selection and help forward our species. Right now, it's f*****, it's all downhill. I love murderers."
A picture of the book was shown to the court with the words "Helter Skelter" written on the cover, a supposed nod to notorious murderer Charles Manson's idea of an apocalyptic war arising from racial tensions.
Two teenage boys have been convicted of conspiracy to murder at Leeds Crown Court after plotting a Columbine-inspired shooting at their school in Northallerton, North Yorkshire.
The teenagers, both 15, sat motionless alongside their tearful mothers as the verdicts were read to them today.
The older boy, wearing a shirt, was also convicted of unlawful wounding, but cleared of a count of aggravated burglary.
During the three-week trial, prosecutors claimed that the pair “hero-worshipped” Eric Harris and Dylan Klebold, the teenagers responsible for murdering 13 people at Columbine High School, Colorado, in 1999.
Jurors heard how the boys had prepared a “hit list” of people they wanted to kill, including fellow students and teachers who had supposedly bullied or wronged them. Analysis of their devices showed that they had researched weapons online and had both downloaded a bomb-making manual.
The older defendant, described as the “leader” of the pair, had supposedly “idolised” Eric Harris, who took up arms with fellow teenager Dylan Klebold and carried out a massacre at Columbine High School, Colorado, killing themselves and 13 others.
The same boy was later found to have kept a diary in which he espoused what prosecutors described as a “far-right wing ideology” and discussed his motivations for wanting to carry out an attack.