Luigi Mangione's Friend Recalls Reading Unabomber Manifesto Together

Luigi Mangione, who is suspected of killing UnitedHealthcare CEO Brian Thompson, may have been a big fan of Unabomber Ted Kaczynski's manifesto ... but didn't let on to his friends about this.

We caught up with RJ Martin, a pal of Mangione's, on "TMZ Live" Tuesday ... where he recounted reading Kaczynski's manifesto, also known as 'Industrial Society and Its Future,' in their book club a couple years ago.

As RJ put it ... he and another member "half-jokingly" suggested they give the controversial work a go for one of their reads -- but noted most members of the club struggled to get through the first couple chapters.

While RJ said the group had "a lively conversation" about the manifesto, he did not remember Mangione being particularly moved by the material.

He added ... "I didn't detect any special passion regarding the manifesto. We read all sorts of different books. In our group text chat, we had a lot of great conversations -- and there were all sorts of things we found fascinating."

RJ emphasized that "nothing stood out about reading that book" when it came to Mangione.

However, the material did seem to resonate with Mangione ... later giving the manifesto a 4-star review on Goodreads, praising it as an "interesting" read.

Still, RJ is unconvinced his friend could be capable of the violence he's accused of, adding ... "The Luigi I know could've never murdered someone. It's just inconceivable."