The third man to accuse Elmo voice actor Kevin Clash of underage sex abuse wrote a book in 2009 about their allegedly illegal trysts -- in which he refers to Clash as "Mr. Tickler."

The accuser's lawyer read an excerpt from the book -- which is yet unpublished -- during a news conference today, quoting, "Mr. Tickler is what I will call him. And the game we played was father and son."

The lawyer read, "When we first met, I was 16 and there was no intercourse. However, lots of heavy kissing and he showed me what it felt like to get on your knees and obey your man."

The dramatic reading continues in even more lurid detail -- seriously ... watch the video at your own discretion.

Clash's lawyer had no comment.

Kevin Clash -- the former voice of Elmo -- used alcohol as a precursor to oral sex with an underage boy ... this according to a new lawsuit that has just been filed by a THIRD accuser.

According to the lawsuit, filed in federal court in Manhattan and obtained by TMZ, the alleged victim met Clash on a gay phone chat line when he was 16 and Clash was approximately 40.

The accuser -- now 28 -- says he was in NYC pursuing modeling opportunities when Clash invited him to his swanky NYC apartment, which was strewn with Elmo dolls and photos of Elmo with famous people, including Beyonce and Tyra Banks. 

During the encounter, the accuser -- who refers to himself as John Doe -- claims Clash plied him with alcohol and "groomed him," and then engaged in sexual contact that included "oral sex and digital penetration of John's anus."

The accuser says Clash stayed in touch with him after the sexual encounter, although no further sexual contact occurred until he became an adult.

The accuser is suing for unspecified damages.

As you know, Clash resigned from Sesame Street hours after the second accuser filed suit.