Samuel Clements (Mark Twain) was an abolitionist. Huck referred to the runaway slave Jim as "nigger Jim" in the first of the book. Many people can't get beyond this to the progression of their relationship where Huck sees Jim as a friend and an equal and refers to him as "Jim".
Precisely! It's an excellent learning tool in teaching anti racism!
You wrote, "Samuel Clements (Mark Twain) was an abolitionist."
Mark Twain's real name was Samuel Clemens. When young, barely a teenager, he was (very briefly) a Confederate soldier. As a full grown man, he would not have been an abolitionist since slavery had already been abolished.
In The Private History of a Campaign that Failed Twain recounts his risible participation in an informal local militia formed to fight for the Confederacy. He was lucky not to have been hanged by a Union colonel named Grant who was rounding up and hanging little guerrilla bands like his. Without benefit of an ACLU Lawyer. It speaks well of his character that he provided financial support for Grant at the end of his life.
In his writings he is extremely sympathetic to Blacks and clearly feels that their talents and strengths are under appreciated, but I don't think he could have been called an abolitionist in the sense of publicly advocating the end of slavery prior to the Emancipation Proclamation.
I read this book to my son when he was 5 or so. Instead of "nigger" I would use "slave" - I'm not even sure I used "black". Anyway, I did note to him the progression from Huck treating him like a slave and then to a friend. What a great story on so many levels!