The 'Tiger King' star details his latest relationship in an eye-opening letter sent from his federal prison in Butner, NC, in which he announces he's a groom-to-be with a man named John Graham -- somebody he was in the pokey with last year and apparently fell head over heels for.

In the correspondence, obtained by TMZ from Joe's attorney Autumn Blackledge, JE says he met John after he was released from solitary, when they were housed in the same unit. It wasn't long before they started seeing each other, he explains, and eventually ... somebody popped the question in August.

As for what John's all about ... well, Joe says he's the polar opposite of his soon-to-be-ex-hubby Dillon Passage -- whom he calls "selfish, younger ... and wanting to be famous."

John, meanwhile, is an old soul -- about 400 years, it seems -- who actually practices witchcraft ... this according to Joe. He describes John as someone who isn't seeking the limelight and who loves Joe for him ... the type of love Joe's never experienced, he says.

Sounds like John's a free man now ... but even so, Joe says they talk constantly -- up to 3 times a day -- and exchange mail nonstop. He says their bond is like the one depicted in 'Twilight' ... adding he still has one of John's shirts so he can smell him in the interim.

As we reported ... Dillon's signaled he's ready to finalize his divorce with Joe, with some conditions -- which Joe here calls "extortion," while noting he too wants to be done with it so he can hurry up and marry John.

Of course, he says he's still trying to have his case heard on appeal ... and believes his latest team of attorneys is going to achieve that with "new evidence" Joe says has been obtained in his favor.