Police seized at least a dozen letters during the raid on Jackson's Neverland ranch last week.
"The district attorney is convinced these letters will be crucial to the case against Jackson," a source close to the investigation said yesterday.
"The boy told investigators about letters and poems and their precise location inside Michael's home. These letters were among the evidence seized, along with videotapes.
"They are very explicit and intimate and show a degree of familiarity. Basically, they appear to be love letters from Michael to the boy."
The accuser, 12-year-old Los Angeles cancer victim ******, also told police Jackson's pet name for him was Rubba.
"The boy first told this to his therapist, then repeated it to police," the source said.
"He said Jackson called him Rubba because one of the games they used to play was called rubba rubba. The boy said, 'Michael told me he was my rubba rubba friend."'
Although police handling the case in Santa Barbara, California, have been barred from revealing any details, it is understood the alleged abuse took place in February.
Jackson is said to have befriended ****** -- who, during a controversial British documentary screened earlier this year, admitted having shared a bed with the singer -- over a long period.
A source close to 45-year-old Jackson said: "For the past five or six years, he's taken kids from fractured homes and nurtured them as a father figure. As they get older, he teaches them fun things to do.
"Michael says he learned this from an adult when he was a teenager. No-one on his staff ever said anything. He's a tyrant. Everybody obeys the man."
Another Jackson source said: "The boy told of everything that went on and described things in Michael's closets, his bathroom, what was under his bed."
In her statements to police, ******'s mother, #####, has revealed that Jackson encouraged her son to call him "Daddy".
##### has claimed she, ****** and his two siblings were held virtual prisoners at Neverland, 100km north of Santa Barbara.
When she learned of her son's accusations of molestation -- which included claims that Jackson plied him with wine and sleeping pills -- she fled with her family in the dead of night.
A source close to district attorney Tom Sneddon, who will prosecute the case, said: "The DA is very confident with the evidence he has. This includes letters, videos and computer files. They believe they have enough to nail Jackson."
Jackson's legal team is said to be encouraging him to consider a plea bargain, possibly an insanity defence that would allow him to serve time in a state mental hospital instead of jail.
There was speculation yesterday that the singer raised his $3 million bail with the help of Miami-based Al Malnik, a lawyer for notorious gangster Meyer Lansky, who died in 1983.
Malnik reportedly began making large loans to Jackson when his career nosedived.
Malnik, 69, was cited by the New Jersey Casino Control Commission in 1980 and 1992 as a person of unsuitable character.
The prosecution source said: "It just gets weirder and weirder. Michael supposedly has financial problems, but hiring someone with Mob connections is bizarre."
Prosecutors are said to be spreading a net around the world for other alleged victims. One boy of special interest is said to be in El Salvador, another in South Africa.
They are also said to be considering quizzing the son of an English premiership footballer.
Charges of aiding and abetting are likely to be filed against members of Jackson's entourage.
At the same time as the Neverland swoop, police raided two homes in Los Angeles.
One was reportedly the residence of Marc Schaffel, a gay video producer who filmed the rebuttal tape Jackson released just after Martin Bashir's British documentary aired.
