A Personal Demon (1985) by David Bischoff, Rich Brown, and Linda Richardson. The front cover blurb:
A college professor conjures up a shapely demon to live within the laws of our world.
Originally published in Fantastic as a series of short stories and novelettes by "Michael F. X. Milhaus". The original stories were: "A Personal Demon" (February 1976, available at the Internet Archive); "In a Pig's Eye" (May 1976, available at the Internet Archive); "With Good Intentions" (September 1977, available at the Internet Archive); "A Trick of the Tail" (December 1977, available at the Internet Archive); and "Where Angels Fear to Tread" (April 1978).
I can't find a copy of the book, but from the magazine story "With Good Intentions", here is a description of Anathae:
Where does one begin a suitable description of this cuddly vixen, this seeming Lolita of Powhattan University who has made Willis Baxter appear her humble Humbert and made him love it? How shall we outline her to you as she stands, at the beginning of this our third saga, in the good Professor's office? Shall we call attention to the flame of tawny red hair which covers her head, framing the storybook beauty of her apparently teenage face, and licks down her classically formed shoulders to end just even with her 18-inch waist? Shall we point out other aspects of her lithesome figure, poking out and in, and curving provocatively and cathartically under the dreamy creamy silk of her dress—surely a prototype of pubescent perfection? Shall we alliterate until we are illiterate, pulse and pound with our prose, purple with our passion over her full lips, full hips, her sparkling eyes and sparkling teeth, her short dress and that which lies beneath?
No again. Rather, let us catch our breath, contain our emotion and explain that this little "teenaged" lady is half-demon, half-human, and older than she looks. Rip off that short silky dress she wears there in Willis Baxter's university office and you will find a cute, very expressive tail curling out from the base of her spine; take off her gleaming calf-high boots and you'll see curly hair leading down to genuine Satan-patented cloven hooves; push back the bangs of her fire-engine hair and you'll note two pert little horns.
And a description of Willis Baxter:
You may think you've seen his type before. The Rumpled Young University Professor With Chalk Dust on His Hands. The Absent-Minded Bumbling Bookish Boob. Or, in a word, a klutz.
But scratch the stereotype and find beneath, the man. As he sits before us now in his new position as chairman of the Arts and Sciences Division of old Powhattan University, let us remember that it was this seemingly meek man who precipitated these adventures by performing a drunken invocation at a party which brought the nubile Anathae to our attention. Let us not forget to recall, dear Reader, that as we closed our last exciting tale—in which we detailed how the Governor of the Commonwealth was transformed into a prime candidate for the bacon factory—the supposedly meek Willis Baxter, for the sake of his beloved Anathae, unflinchingly faced The Dark One Himself.