I'm going to list series as single entries as when I start on one I generally don't stop.

Gordon R Dickson: The Dragon and the George
Tolkien: Hobbit/LOTR
Weis/Hickman: Dragonlance
CS Lewis: Narnia series
Douglas Adams: HHGTTG
Bruce Carter: The Perilous Descent
Colin Thiele: Storm Boy/Fire in the Stone/Albatross Two (Met him when I was very young when he visited our school too!)
Terry Pratchett: Discworld series - no actually, dammit, all of them.
Anne McCaffery: Dragon Rider series
Jim Butcher: Dresden Files
The Bible
Petr Beckmann: A History of Pi
George R R Martin: Wild Cards
Stephen King: Christine
Mercedes Lackey: Dragon's Bane
David Gemmel: Legend (first of the Druss series)