"The Time Machine" by Herbert George Wells

This is a relatively short book, though I 'read' it in the audio version, which wasn't the best, as the voice volume would go up and down (bad post-production/equalization) which is pretty sucky for commuting purposes on the train.

H.G. Wells was one of the most prolific authors of all time and in the most varied plethora of subjects.  The Time Machine is a classic of world literature, and together with War of the Worlds, it helped define the genre of Science Fiction. 

In the novel, the unnamed time traveler builds a time machine (with cogs and levers) and travels 800,000 years in the future.  Our planet resembles a garden, with many scattered ruins of previous civilization, but seemingly inhabited by small, blonde, smiling little people who only frolic around all day long, eat fruit, laugh, dance, place, make love and sleep.  The time traveler calls these happy creatures Elloi.  Only later he discovers that there's another race of post-humans, which he calls Morlocks, who live underground, look like albino vampires, and provide everything for the Elloi above ground until they are fat and ready - wait for it - to be eaten!  Apparently all other animal species died out all over the planet, so the Morlocks use the Elloi as their meat source (the Elloi being strictly frutarian). 

The obvious social commentary on upper and lower classes in the XIX century English society is obvious, however it does not detract from the enjoyment of the story by itself.  The time traveler barely escapes the cannibalistic Morlocks and travels even further in the future when the planet is covered by an infinite ocean and populated only by intelligent crabs.

Eventually the time traveler comes back to London, but his friends do not believe him, and after a few days of depression and frustration he leaves forever in his time machine. 

The Time Machine is probably one of the most copied books, both plot and characters, in the history of literature.  It has served as a basis of so many other works of art that they are too many to count.  In recent years it is the underlying inspiration for the Traditional Humans and the Calibans in Dan Simmons' Ilium and Olympos dilogy.