Let me start by admitting that I'm not a fan of The Adventures of Huckleberry Finn and recall very few of its plot points. I generally don't care for picaresque novels and didn't find the comic elements humorous.
James tells the same story from the point of view of the runaway slave Jim. The gimmick is that Jim is extremely well educated and merely plays the fool for the benefit of his white oppressors. The first few chapters show Jim teaching young black children "the correct incorrect grammar" for the "slave filter," and several plot developments involve white folks being confused when they hear a slave speak "proper" English.
It is not surprising that James focuses on language given that the vernacular is the most notable aspect of Twain's book and of Everett's earlier book Erasure (filmed as American Fiction).
Everett leans too heavily on his themes. Not only can Jim read and write, he is inexplicably well educated. He has a dream in which he argues with Voltaire about natural versus civil liberties, and has visions of John Locke. When alone, the slaves have explicit conversations about their predicament. At the same time, Everett skims past the adventures, such as when Huck and Jim explore a house that comes floating down the river.
The story comes alive once it deviates from Huckleberry Finn. The ironic humor, the action, and the intensity all increase during the period when Huck and Jim are separated. When they are reunited, it is Jim who drives the action, not Huck. The final chapters are completely different in both incident and tone: Huckleberry Finn ends with controversial chapters that "devolve into little more than minstrel-show satire and broad comedy" (says Ernest Hemingway); James ends with violence and righteous anger.