Start with all the Tamora Pierce ones! I suppose that's fairly obvious, but here's some other books that people on the list have recommended as enjoyable.

Fantasy

Terry Brooks
The Voyage of the Jerle Shannara (trilogy)(Ilsewitch; Antrax; and Morgawr), Magic Kingdom for Sale--Sold!

Lynn Flewelling
Luck in the Shadows, Stalking Darkness, Traitor's Moon (Nightrunner books), The Bone Doll's Twin

Maggie Furey
Aurian, Harp of Wind, Sword of Flame and Dhiamara

Mary Gentle
Ash: A Secret History, Grunts (especially if you RPG), Rats & Gargoyles

Elizabeth Hand
Waking the Moon, Black Light (and Winterlong and Glimmering, which are more strictly SF)

CS Lewis
Narnia (Magician's Nephew, Lion Witch and Wardrobe, etc.)

Gregory Maguire
Wicked

Juliet McKenna
Thief's Gamble, Swordsman's Oath, Gambler's Fortune, Assasin's Edge and Warrior's Bond

China Mieville
King Rat, Perdito Street Station

Elizabeth Moon
Deed of Paksenarrion: Sheepfarmer's Daughter, Divided Allegience and Oath of Gold

Meredith Pierce
Darkangel Trilogy(Darkangel; A Gathering of Gargoyles; the Pearl of the Soul of the World.)

Philip Pullman
His Dark Materials (trilogy - Northern Lights, the Subtle Knife and The Amber Spyglass), The Clockwork Princess

JK Rowling
The Harry Potter books

Sharon Shinn
Archangel Trilogy(Archangel; Jovah's Angel; the Alleluia Files), The ShapeChanger's Wife, Summers at Castle Auburn

JRR Tolkien
The Lord of the Rings, The Hobbit

TH White
The Once and Future King

Other Genres

Richard Adams
Watership Down, Tales From Watership Down (Animal)

Maya Angelou
I Know Why The Caged Bird Sings (Auto-biography)

Jane Austen
Mansfield Park, Pride & Prejudice (Classic)

Mikhael Bulgakov
The Master and Margarita (Classic/Horror)

Caleb Carr
The Alienist, Angel of Darkness (Mystery/Horror)

Paula Danziger
The Amber Brown Series: Amber Brown is Not a Crayon; Amber Brown is Feeling Blue; Amber Brown is Seeing Red; You Can't Eat Your Chicken Pox, Amber Brown; Amber Brown Wants Extra Credit; It's Justin Time, Amber Brown (Childrens)

Lois Duncan
Locked in Time (children's)

Janet Evanovich
The Stephanie Plum Mysteries: One for the Money; Two for the Dough; Thre to Get Deadly; Four to Score; High Five; Hot Six; Seven Up (Crime)

Laurel K. Hamilton
Anita Blake: Gulity PLeasure, Laughing Corpse, Circus of the Damned, The Lunatic Cafe (Horror)

Daniel Handler
The Basic Eight, Shut Your Mouth (Humor/Horror)

Lemony Snicket (aka Daniel Handler)
A Series of Unfortunate Events thus far: The Bad Beginning; The Reptile Room; the Wide Window; the Miserable Mill; the Austere Academy; the Ersatz Elevator; the Vile Village; The Hostile Hospital (childrens)

E. Lynn Harris
Invisible Life (Fiction)

Armistead Maupin
Tales of the City (Fiction)

Phylis Reynolds Naylor
the Alice Series: the Agony of Alice; Alice in Lace; Alice In-between; Alice on the Outside; Achingly Alice; Alice in April; Simply Alice; Alice Alone; All But Alice; the Grooming of Alice; Alice the Brave; Outrageously Alice; Alice in Rapure, Sort of; Reluctantly Alice (Childrens)

Christopher Rice
The Snow Garden, A Density of Souls (Mystery/Fiction)

Murasaki Shikubu
The Tale of Genji (Classic)

Sharon Shinn
Heart of Gold, Wrapt in Crystal (SF)