I have been reading this collection of Twain's principle works on and off for years now. Every now and again I read another novel contained therein.
The Adventures Of Tom Sawyer, The Adventures Of Huckleberry Finn:
These two adventure novels are Twain's best known works. There is some lovely descriptive prose here, and some very amusing episodes, too. These remain a good read today.
One fact struck me unexpectedly: the characters in these novels are hugely superstitious. On almost every page is some silly superstition held by Tom or Huck described.
The Prince And The Pauper:
I was constantly surprised by the readability of this book. The story develops apace, and though we have all probably read or heard or seen tales of the rich and privileged swapping places with the poor, Twain's tale remains fresh and entertaining. I enjoyed this book more than Tom Sawyer and Huck Finn!
It was a surprisingly serious work, using the "reversal of fortunes" device to finely dissect the nature of human folly, greed, ignorance, and entitlement. I would think this work to be far more accessible to the young reader than the others, too.
A Connecticut Yankee In King Arthur's Court:
This was an increasingly impressive book. It has some hugely humorous moments and is the most politically and religiously outspoken of Twain's writing I have yet read. Moments in this book reminded me of Dickens, with its socially conscious message and masterly command of prose. Some truly wicked humour features here. I cannot decide if I enjoyed this more than The Prince And The Pauper. This is the more serious book in some ways, yet it does feature craftily mordant humour. So, this is more "grown up" than Pauper, but the latter still probably pips it in its easy mix of humour, social commentary, and sheer readability.