Crystal Hefner -- who was Hugh's 3rd and final wife, and who was still married to him when he died in 2017 -- is coming out with a new memoir soon ... and in it, she kinda trashes her late husband, not to mention the culture she says was cultivated in his Playboy world.

The book's called "Only Say Good Things: Surviving Playboy and Finding Myself" -- and the title, she says, is based on what she claims HH asked to say about him when he was gone.

While Crystal did do that in the immediate aftermath of his passing, she's now singing a different tune -- looking back at their relationship with "regret and disgust" ... as the NYT put it in a new profile. As for why she's flip-flopping, Crystal suggests she was under a spell.

She says, "At the time I must’ve been brainwashed or something." There are a lot of allegations she's floating about how Hugh -- including claims that he was strict about her appearance/weight, not to mention making sure she kept to a curfew he'd set for her.

Crystal even suggests she was somewhat trapped at the Playboy Mansion -- saying she was able to "escape" one time by telling a guard she had to go buy tampons ... but then returned on her own volition because she says she had Stockholm syndrome at the time.

BTW, she isn't the first Playboy model to speak out against Hugh and the company -- in recent years, others have come forward with similar claims ... including Kendra Wilkinson, who just recently revealed she was seriously traumatized from her time with Playboy/Hugh.