The Hollow Hills is a novel

Novel

A novel is a book of long narrative in literary prose. The genre has historical roots both in the fields of the medieval and early modern romance and in the tradition of the novella. The latter supplied the present generic term in the late 18th century....

 by Mary Stewart

Mary Stewart

Mary Florence Elinor Stewart is a popular English novelist, best known for her Merlin series, which straddles the boundary between the historical novel and the fantasy genre.-Career:...

. It is the second in a quintet of novels covering the Arthurian Legends. This book is preceded by The Crystal Cave

The Crystal Cave

The Crystal Cave is a 1970 fantasy novel by Mary Stewart. The first in a quintet of novels covering the Arthurian legend, it is followed by The Hollow Hills.-Plot introduction:...

and succeeded by The Last Enchantment

The Last Enchantment

The Last Enchantment is a 1979 fantasy novel by Mary Stewart. It is the third in a quintet of novels covering the Arthurian legend, preceded by The Hollow Hills and succeeded by The Wicked Day.-Plot introduction:...

. The Hollow Hills was written in 1970 and published in 1973.
The protagonist

Protagonist

A protagonist is the main character of a literary, theatrical, cinematic, or musical narrative, around whom the events of the narrative's plot revolve and with whom the audience is intended to most identify...

 in this story is Merlin

Merlin

Merlin is a legendary figure best known as the wizard featured in the Arthurian legend. The standard depiction of the character first appears in Geoffrey of Monmouth's Historia Regum Britanniae, written c. 1136, and is based on an amalgamation of previous historical and legendary figures...

, the legendary wizard, here depicted as an ordinary mortal but one with extreme creative intelligence and occasional clairvoyance

Clairvoyance

The term clairvoyance is used to refer to the ability to gain information about an object, person, location or physical event through means other than the known human senses, a form of extra-sensory perception...

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