Title: The Neverending Story (My kind of book...)
Genre: Very clever fantasy/philosophy
Length of Book: Umm... It's not very long.
Series or one book: One book split into two films
Details: A story within a story. A "fat little boy of ten or twelve" steals a book that - quite literally - takes him on an adventure through a world built on the dreams of humanity. Originally written in German by Michael Ende (Ironically).
Extra: So meaningful that it would take a good couple of lifetimes to truly analyse. One of the best storylines that I have ever happened across.
Title: Animal Farm
Genre: Dystopian/Political Fiction
Length of Book: Very short. I read it in a few hours.
Series or one book: One book, but there is also a film which I highly recommend.
Details: A group of farm animals rebel against the oppressive farmers and form a society in which all are equal... However it is soon discovered that some are more equal than others.
Extra: George Orwell was a genius... And manages to show how the poles of the political spectrum are far closer together than either side would want to admit.
Title: A Song of Ice and Fire (Can't believe nobody has mentioned this yet)
Genre: "Epic Fantasy"
Length of Book: Very long... And there are lots of 'em.
Series or one book: A huuuuge series. There is a tie-in TV show produced by HBO which is brilliant.
Details: A clash of opposing forces in the Seven Kingdoms of Westeros and the surrounding lands, houses fight for land, titles and revenge whilst the dead walk in the North and dragons are rising in the East. In a word: spectacular.
Extra: As much as I loathe to admit it, I find these books more gripping than Lord of the Rings, which (if you haven't read it) is utterly fantastic.
Title: His Dark Materials (Better known as The Golden Compass)
Genre: Alternate Reality/Dark Fantasy
Length of Book: Not too long.
Series or one book: Three books getting gradually more complex.
Details: Set in a parallel world fraught with the struggles between science and religion.
Extra: Scary. Some of the connotations and concepts in the books are pretty complex.
Title: The Wave
Genre: Social Experiment - based on real events
Length of Book: Very short.
Series or one book: One book. There is also a film out in German with English subtitles, although it doesn't quite stick to the story as much.
Details: A high school experiment spirals out of control as a dictatorship forms in a classroom.
Extra: Based on real events in a high school in the USA where an experiment was carried out to mimic how people were indoctrinated by Nazi beliefs during and prior to the Second World War. Psychologically scary.
I also highly recommend LotR and the Harry Potter books, although I would have hoped most people would know what those are about.