Alright, while I do have to admit that I hate that word as well and get uncomfortable with reading any book with the word in particular. You have to realise that the book was written before the Civil War, which viewed that word as okay to say at the time.
Also you really see Huck be really conflicted as he wishes to tell somebody about what Jim is doing (he wants to run away from the woman who plans on selling him for 800 dollars. You also see Huck become friends with Jim and go on an adventure with him... his views on slavery and people changes and continue to change throughout the novel. It's really the thought of the book that counts and not the word itself. While yes the word does have an effect on the story plot in general it actually takes centre stage for Huck's adventure with Jim.
I do not agree with you on the part that the novel is racist. It is just expressing human freedom during a time when some of what the book was talking about was frowned upon. Also racism hurts everybody. It isn't fair but during the course of this book it sets the reader on an adventure and tells a story of the cost of human freedom.
That is what Huckleberry Finn is about.