I don't know if this is a complete list or not, but I tried.

The Prydain Chronicles
The Book of Three
The Black Cauldron
The Castle of Llyr
Taran Wanderer
The High King
The Foundling

The Westmark Trilogy
Westmark
The Kestrel
The Beggar Queen

The Vesper Holly Adventures
The Illyrian Adventure
The El Dorado Adventure
The Drackenburg Adventure
The Jedera Adventure
The Philadelphia Adventure

Others
The Iron Ring
The Arkadians
The Fortune-Tellers
The Remarkable Journeys of Prince Jen
The First Two Lives of Lukas-Kasha
The Town Cats and Other Tales
The Wizard in the Tree
The Cat Who Wished to Be a Man
The Marvelous Misadventures of Sebastian
Time Cat
Gypsy Rizka
The House Gobbaleen
Coll and His White Pig
Truthful Harp