Luigi Mangione Claimed He Had $6 Million From His Family While on Vacation

Luigi Mangione mentioned the big bucks he had in the bank on a trip to Thailand ... and also, repeatedly brought up a couple of revealing books -- TMZ has learned.

A source -- who doesn't want to be identified by name, but who spent significant time with Luigi back in April -- tells TMZ ... the two went to the popular tourism destination together, hitting up bars and visiting a weed shop, and cracking inside jokes over a period of two days.

According to his friend, Luigi was "making lawyer money" coding for TrueCar ... a telling statement because Mangione reportedly stopped working for them months before going to Thailand.

He told our source he had $6 million in the bank, all of it coming from his family. Talking about his family seemed to be a sore subject, according to the friend, so he didn't press Luigi on it.

Our source says Mangione might have mentioned owning a 3-D printer too -- remember, it's been reported police believe the gun he allegedly had in his possession when he was arrested was 3-D printed.

Our source says Luigi mentioned 2 books repeatedly during their days together -- "The Secret Assassin" by Joseph Conrad and "Confessions of an Economic Hitman" by John Perkins.

The former is a well-known classic in English literature ... focused on a character who commits a bombing to convince the British government to quash Anarchist leanings before he himself is fatally stabbed.

The latter book is a semi-autobiographical book published by Perkins about his time working for the engineering consulting firm Chas. T. Main ... which claims he went to underdeveloped countries with loans to build infrastructure -- which helped essentially trap those who took the loans into a system of American control.

Our source says he also read Perkins' book ... and, the two discussed it in detail -- talking about how they both enjoyed takeaways from it.

Luigi and our source last spoke in July ... when we're told Luigi blasted the U.S. healthcare system and talked about the need for universal healthcare.

We're told he talked about needing change in general -- and didn't specifically point to any health issue he was having -- including the back injury that's been a topic of discussion.

We're told he only brought up his back once on the trip -- saying he was hopeful that hot springs in Japan would feel soothing.