Question: What novel by James H. Schmitz involves a type of theoretical or fictional entity existing in a dimension beyond our own?

Context articles: Kaluza–Klein theory
Agent of Vega
The Demon Breed
Trigger Argee
Interdimensional being
Telzey Amberdon
The Witches of Karres
The Wizard of Karres
David St. Hubbins
Phoenix Force (comics)

In physics, Kaluza–Klein theory (KK theory) is a unified field theory of gravitation and electromagnetism built around the idea of a fifth dimension beyond the usual four of space and time.
 It is considered to be an important precursor to string theory.
Agent of Vega is a science fiction novel by American writer James H. Schmitz, published in 1960.
 It is a collection of stories that originally appeared separately in magazines.
 It was republished in 2001 as "Agent of Vega & Other Stories".
The Demon Breed is a 1968 science fiction novel by James H. Schmitz, originally serialized in "Analog" in a shorter form as "The Tuvela".
 It was first published in paperback in the Ace Science Fiction Specials line, with a Science Fiction Book Club edition following in 1969.
 MacDonald & Co. issued a British hardcover the same year, reprinting it as a Futura paperback in 1974.
 A Dutch translation, "Des Duivels", appeared in 1971, and a French translation, "Race démoniaque", in 1973.
 Ace reissued its edition in 1979 and 1981.
 In 2001, Baen Books compiled the novel in its paperback omnibus "The Hub: Dangerous Territory".
Trigger Argee is a fictional character in a series of science fiction short stories by James H. Schmitz, starting from 1958.
 As her first name implies, she inherited the ability to be a crack shot with the sidearms of her day from her father.
 In the novel "A Tale of Two Clocks" she even operates powered armor in an assault upon alien beings which have infested a spaceship.
 Instrumental in the discovery of a hidden alien race, the Old Galactics, she serves as a secret agent of the Hub Overgovernment.
 Eventually Trigger teams up with a younger woman, Telzey Amberdon of the fictional planet Orado.
 The stories were originally published in "Analog Science Fiction and Science Fact".
 The series was quite popular.
 Some issues had covers featuring this character.
An extradimensional being or intelligence (also intra-dimensional and other-dimensional) is a type of theoretical or fictional entity existing in a dimension beyond our own.
 Such beings are common in science fiction, fantasy and the supernatural.
Telzey Amberdon is a fictional character in a series of science fiction short stories and two short novels by James H. Schmitz, taking place in his "Federation of the Hub" fictional universe, presumably in the mid-4th millennium.
 She is introduced as a fifteen-year-old genius, a first-year law student, living on the human-settled planet Orado (whose name comes from Eldorado by a pun).
 Through interaction with alien psychic animals on a resort planet, she discovers that she has psychic powers.
 Upon her return to her home planet, her abilities are recognized by a mechanism at the spaceport reentry gate and she is effectively made an agent of the Psychology Service.
The Witches of Karres is a space opera novel by James H. Schmitz.
 It deals with a young space ship captain who finds himself increasingly embroiled in wild adventures involving interdimensional alien invaders, space pirates, and magic power.
 The story is unrelated to the "Hub" series of stories by Schmitz.
The Wizard of Karres is a novel by Mercedes Lackey, Eric Flint, and Dave Freer that was published by Baen Books in 2004, as a sequel to The Witches of Karres by James H. Schmitz.
David Ivor St. Hubbins is a fictional character in the mockumentary film "This Is Spinal Tap" (1984).
 In the film, he is the lead singer and rhythm guitarist of the mock rock band Spinal Tap.
 David is played by actor Michael McKean, who improvised the role through the whole film.
 McKean writes in his introduction to "This Is Spinal Tap: The Official Guide", "When I am called upon to generate copy about the mostly fictional entity called Spinal Tap, I usually do so in the mostly fictional character of David St. Hubbins..."
The Phoenix Force is a fictional entity appearing in American comic books published by Marvel Comics.
 The entity has bonded with other characters, and often used the alias Phoenix while bonded.