Harry Potter is AMAZING. The movies spread word of this incredible magical world even further, but it's a bit sad that there's an entire universe of Potter magic that only book-readers know about. It's a universe of stories and characters that didn't make it into the movies.
Here's a list of my favorite awesome, magical things that only came to life on J.K. Rowling's wonderful pages...
1. Hermione's Indefatigable Logic
The films show her as studious, but in the books she's practically an actual genius. On their way to the Sorcerer's Stone - when Ron plays Chess and Harry catches the keys - Hermione solves a Very Difficult Riddle, saving them all from drinking poison. Because she's awesome.
2. The Hogwarts Ghosts
The Harry Potter movies tell you about Nearly-Headless Nick (a.k.a. Nicholas de Mimsy-Porpington), but less is explained about the other ghosts. Significantly, the Slytherin Ghost (The Bloody Baron) killed the Ravenclaw Ghost (Helena Ravenclaw a.k.a. The Grey Lady) in a jealous rage borne of unrequited love. The Hufflepuff Ghost - The Fat Friar - was a large, religious man, who was executed when Muggles became suspicious of his magical healing powers.
3. Detailed Dark Arts
There are a lot of Dark Arts that are explained more deeply in the Harry Potter books. The practice of Necromancy - reanimating dead creatures to do one's bidding - creates the Inferi in the Wizarding World. These are the slimy, overly-handsy corpses that go after Dumbledore and Harry in The Half-Blood Prince which - according to J.K. Rowling, are NOT zombies!
4. S.P.E.W.: Hermione's House Elf Crusade
Hermione gets politically active in the later books, establishing the Society for the Protection of Elfish Welfare, an advocacy group for House Elf rights. In spite of being mocked, Hermione's insistence on compassion to House Elves is central to some major plot points - Harry learns more about the Horcrux locket from Kreacher, a house elf.
5. The Magical Roofie
Voldemort's Mom - the pureblood witch, Merope Gaunt - magically roofie'd his father in order to conceive the young Tom Riddle. She uses a 'love potion,' although J.K. Rowling tells us that real love cannot be fabricated. When Voldy's father wakes up from his bewitched state, he runs away from this mad pregnant sex-offender witch. Voldemort reacts reasonably and murders his entire family.
6. Dumbledore's Full, Tragic Backstory
Dumbledore's story makes his success and goodness so much more incredible. At a young age, his little sister, Arianna, was driven mad by an attack by three muggle kids. She then killed her mother in a magical frenzy. Arianna was eventually killed by accident, during a duel between Dumbledore, his slightly mad brother and the dark wizard, Grindelwald.
7. Neville's Almost-Destiny
Oh, man. This one is KILLER. So, it was prophesied that Voldemort would be defeated by an infant - which could have been Harry Potter or Neville Longbottom. The prophecy - made by Professor Trelawney - stated that the child to depose Voldemort would be male, born at the end of July, 1980, and have parents who thrice defied The Dark Lord.
Voldemort chose Harry's destiny for him when he attacked him personally, leaving him with the lightning bolt scar, but The Boy Who Lived could very well have been Neville Longbottom...amazing, huh?
More house elves! Although, Winky's tale is a horribly sad and depressing one. Winky was a House Elf to the Crouch family, caring for Death Eater, Barty Crouch Jr., when his father freed him from Azkaban under the Imperius Curse. Winky was blamed when Barty Jr. cast the Dark Mark at the Quidditch World Cup, and was forcibly freed, subsequently falling into alcoholic despondency. Bad times.
9. Peeves the Poltergeist
Irritation is made manifest in Peeves, the full-scale annoying poltergeist who pops out at students around Hogwarts. He does things like shut Mrs. Norris inside a suit of armor, stuff gum into keyholes and - my personal favorite - drop an entire bag of tarantulas over the breakfast table in the Great Hall. The late, great Rik Mayall actually filmed scenes as Peeves - which were cut from the final movie.