Mantle is one of the greatest baseball players of all-time ... he was a 20x All-Star, 3x AL MVP, and 7x World Series champion.

However, he also had a dark side ... battling alcoholism for decades. In 1995, Mantle received a liver transplant ... but only lived several months longer.

Now, an incredible note written by the Yankees slugger just hit the auction block at Lelands ... and what Mick wrote is pretty powerful.

"I want to dedicate this book to all my friends. Old, new & especially young friends. Please don’t do alcohol & drugs. I never did drugs but alcohol hurt my career terribly," Mantle wrote.

Lelands says it's unclear exactly who or what the letter was for ... but they've authenticated the piece, and it's legit.

Mickey also seems to answer an age-old debate -- who was a better ballplayer -- Willie Mays, Duke Snider or MM.

"People used to ask who was better? Willie, Mickey or Duke. I think you have to look at final statistics and Willie was far ahead at the end."

Mickey continued ... "Guys who took good care of themselves like Willie, Hank Aaron, Stan Musial, Ted Williams, Joe [DiMaggio], etc., were the ones at the top of the final stats."

Mantle -- one of the best 15 baseball players ever -- says he could've actually been even more lethal at the plate and in the field if he'd laid off the booze.

"I had taken better care of myself I could have done a lot better. I didn’t even take care or do my rehab on my legs like I was supposed to. I drank too much when I was playing from 1952, after my dad died till 1969. After I retired in '69, I drank more and more (I’ll always be ashamed of)."

FYI -- Mantle badly tore up his knee when his cleat got stuck on a drain in the outfield while pursuing a fly ball hit by Mays in October, 1951.

"So to everyone out there especially young people, little leaguers etc. 'Don’t end up over the hill before you even start to climb it.' Makes us all proud of you. Don’t do drugs & alcohol.”