Cooper's The Dark is Rising sequence, Pratchett's Discworld novels, Pullman's His Dark Materials trilogy, and I suspect that Holland's Sistersong will make the list once it's old enough for the tag "of all time".
Orlando - Virgina Woolf
Earthsea (all of it) - Ursula K. Le Guin
A Night in the Lonesome October - Roger Zelazny
Jonathan Strange and Mr Norell - Suzanna Clarke
The Mysterious Stranger - Mark Twain
The Bas Lag trilogy - China Mieville
Lud-in-the-Mist - Hope Mirrlees
Redwall (most of them) - Brian Jacques
Discworld (most of them) - Terry Pratchett
His Dark Materials - Philip Pullman
The Saint of Bright Doors - Vajra Chandrasekera
Nights at the Circus - Angela Carter
I'm missing loads, I know... more to come likely...
If you're accepting humour, Pratchett's Discworld series, especially Mort and Small Gods. If not, the Weis & Hickman Dragonlance books - the dragon and twins trilogies specifically - and their non-Dragonlance 7 book Death Gate Cycle.
The Broken Sword - Poul Anderson
The Dweller in the Mirage - A.E. Merritt
The Drawing of the Dark - Tim Powers
The Hollow Hills - Mary Stewart
Mythago Wood - Robert Holdstock
Comments
Orlando - Virgina Woolf
Earthsea (all of it) - Ursula K. Le Guin
A Night in the Lonesome October - Roger Zelazny
Jonathan Strange and Mr Norell - Suzanna Clarke
The Mysterious Stranger - Mark Twain
The Bas Lag trilogy - China Mieville
Lud-in-the-Mist - Hope Mirrlees
Discworld (most of them) - Terry Pratchett
His Dark Materials - Philip Pullman
The Saint of Bright Doors - Vajra Chandrasekera
Nights at the Circus - Angela Carter
I'm missing loads, I know... more to come likely...
The Dweller in the Mirage - A.E. Merritt
The Drawing of the Dark - Tim Powers
The Hollow Hills - Mary Stewart
Mythago Wood - Robert Holdstock