Initially, it was tough going because most of the book is written in a dialectical form due to the fact that Huck himself is the narrarator. In case you've been living under an even bigger rock than me, here's the summary: Huck Finn and the Nigger Jim run away from home, Huck from his father and Jim from his mmaster and slavery in general. They build a raft to float up the Mississippi, and along the way, hijinks ensue.
The forefront issue always mentioned in regards to this book is race, and I confess, I was surprised by the constant usage of the word “nigger.” It's hard for me to see Huck Finn as being entirely a childrens' book when considering that, but, on the other hand, I am looking at it from a very 20th Century perspective. It's interesting too that, unless you simply take extreme umbrage to the word itself, there's nothing racist in Huck. Indeed, Jim is nobler and kinder than anyone else in the novel, including Huck himself.
The best part of the book concerns the Duke and the Dauphin, certainly two of the greatest comic villians in all of literature. The civil war beween two families is also interesting, mostly in that it points out that, even though neither one of the families can remember the slight that sarted the fight, both are willing to keep on killing until kingdom come.
And, I guess those are my thoughts on Huck Finn. Oh yeah, one more thing, I doubt this would be considered a childrens' book at all if it didn't feature Tom Sawyer in the beginning and end. It's a much more diverse and mature work than Tom Sawyer, Great Illustrated Classic.