This is an old and seemingly obscure (I had to add it to the Goodreads database myself) volume of a book series called The Complete Works of Mark Twain. The Mysterious Stranger was his final, unfinished book, published after his death in a version that according to wikipedia turned out to be stitched together with pieces added by another author.

The Mysterious Stranger is a rather interesting philosophical novel in which a supernatural boy named Satan disrupts a small town. While Satan does many things that seem cruel, Twain's point of view is that he is simply beyond that petty, foolish human mind. It's a very cynical novel, but interesting.

Also in the collection is A Horse's Tale, which was too dull to get far in, Extract from Captain Stormfield's Visit to Heaven, another rather philosophical story that has an interesting view of heaven that considers what its scale would really mean but that gets bogged down in exposition, several forgettable short stories and one very amusing one - The McWilliamses and the Burglar Alarm - which is silly light humor that doesn't fit with the rest of the book but is the most enjoyable thing in it.

For the most part, this is more interesting than entertaining, but Twain had an interesting way of looking at the world.