Try these:
Fantasy:
Neil Gaiman is an amazing author! Any of his books are good. Try Neverwhere, Stardust, or Coraline.
Any books by Scott Westerfeld or Neal Shusterman are really good too.
Any books by Amelia Atwater-Rhodes are amazing! There is In The Forest of The Night, Demon In My View, Shattered Mirror, Midnight Predator. And then there is the The Kiesha'ra series: Hawksong, Snakecharm, Falcondance, Wolfcry and Wyverhail.
Abarat by Clive Barker. Make sure you get the hardcover version though!!! If you liked the first one be sure to check out the next book in the series Abarat: Days of Magic, Nights of War.
Daughters of Destiny series: Keeper of the Winds, Keeper of the Waters, Keeper of the Flames and Keeper of the Earth by Jenna Solitaire.
His Dark Materials trilogy (The Golden Compass, The Subtle Knife and The Amber Spyglass) by Philip Pullman
The Cry of the Icemark by Stuart Hill
The Sight and Fell by David Clement-Davies
Poison Study by Maria V. Snyder
Howl’s Moving Castle by Dianna Wynn Jones
Inkheart, Inkspell and Inkdeath by Cornelia Funke
The Wind Singer, Slaves of the Mastery and Firesong by William Nicholson
The Earthsea series (starting with A Wizard of Earthsea) by Ursula K. Le Guin
Shadow Spinner by Susan Fletcher
The Inheritance Trilogy by Christopher Paolini
Harry Potter series by J.K. Rowling
The Chronicles of Narnia (7 books) by C.S. Lewis
The Hobbit and The Lord of the Rings trilogy by J.R.R. Tolkien
The Prophecy of the Stones by Flavia Bujor
Old Magic or The Named series by Marianne Curley
Ender’s Game series by Orson Scott Card
Science Fiction:
The Maximum Ride series (The Angel Experiment, Schools Out Forever, Saving the World and Other Extreme Sports, The Final Warning) by James Patterson
Uglies series by Scott Westerfeld
Peeps and The Last Days by Scott Westerfeld
The Secret Under My Skin by Janet McNaughton
Unwind by Neal Shusterman
Horror/Thriller:
Almost any books by Stephen King are great for horror.
The Haunting of Alaizabel Cray or Poison by Chris Wooding
The Abhorsen Trilogy (Sabriel, Lirael, Abhorsen) by Garth Nix
Blood and Chocolate by Annette Curtis Klause
Jurassic Park by Michael Crichton
Wake by Lisa McMann
Project 17 by Laurie Stolarz
Romance:
William Shakespeare is good for romance, but he has written many comedies and tragedies too.
Any of Jane Austen’s books are guaranteed to have romance in them.
How To Kill A Rock Star by Tiffanie Debartolo
The Princess Bride by William Goldman
Nick and Norah’s Infinite Playlist by Rachel Cohn and David Levithan
Tuck Everlasting by Natalie Babbitt
A Great and Terrible Beauty, Rebel Angels and The Far Sweet Thing by Libba Bray
The Morganville Vampire series (Glass Houses, Dead Girls Dance, Midnight Alley and Feast of Fools) by Rachel Cain
Beauty: A Retelling of the Story of Beauty and the Beast by Robin Mckinley
Wuthering Heights by Charlotte Bronte
Twilight series by Stephanie Meyer
Realistic Fiction/Action:
13 Little Blue Envelopes and Girl At Sea by Maureen Johnson
Crank, Burned, Impulse, and Identical by Ellen Hopkins
The Secret Life of Bees by Sue Monk Kidd
So Yesterday by Scott Westernfeld
The Sisterhood of the Traveling Pants series by Ann Brashares
Dreamland by Sarah Dessen
The Lovely Bones by Alice Sebold
All-American Girl by Meg Cabot
A Mango Shaped Space by Wendy Mass
The Perks of Being a Wallflower by Stephen Chbosky
Go Ask Alice by Anonymous
Paper Towns, Looking For Alaska, and An Abundance of Katherines by John Green
Define “Normal” by Julie Ann Peters
Speak by Laurie Halse Anderson
Tuesdays With Morrie by Mitch Albom
Holes by Louis Sanchar
The Alex Rider series (starting with Stormbreaker) by Anthony Horowitz
Thieves Like Us and Thieves Till We Die by Stephen Cole
13 Reasons Why by Jay Asher
Mystery/Historical Fiction:
Edgar Allen Poe is a really good author. Check out his poems and short stories.
Any books by Agatha Christie are great for a mystery.
Any of the Amelia Peabody mysteries by Elizabeth Peters. (I really like the novels set in Egypt, starting with Crocodile in the Sandbank.)
Any of the Forensic Mysteries by Alane Ferguson. (Starting with The Christopher Killer)
The Riddles of Epsilon by Christine Morton-Shaw
A Northern Light by Jennifer Donnelly
Water For Elephants by Sara Gruen
The Thirteenth Tale by Diane Setterfield
The Kite Runner by Khaled Hosseini
The Da Vinci Code by Dan Brown