This is a series of three fantasy books written by Meredith Ann Pierce, unrelated to the television series of a similar title.
Books in this series include:
In the publication I have from 1990, these three books were compiled into one hardcover simply called The Darkangel Trilogy. The usage of "darkangel" as one compound word is consistent throughout the series. The world and tone are distinctly fantastical, although the setting can be very technically described as science fiction, given the loosest possible definition of the term. The people in this story live on Earth's moon, although this is not explicitly stated as much as very obviously implied. The characters reference a planet in the sky called Oceanus, which occasionally eclipses over their sun called Solstar, and many references are made to The Ancient Ones who terraformed the world into what it is now.
When I say the loosest possible definition of science fiction, I mean that this is about as far as the science goes. The characters count time in units of "day-months," and there are other oblique references to the planetary patterns involved, but beyond that, this is distinctly a fantasy series. I personally find this very funny, and also a little bit refreshing and fun. There are grand mythical creatures, magic, prophesy, all the fantastical tropes of a fairy tale--and they can be traced back to some totally-not-witch who lived in a dome city. There's a pegasus. He was created by an "Ancient" named Ravenna. This is not explained. They take it extremely seriously.
When I say that it is distinctly fantasy, I mean that even beyond the literal appearance of things like magic, potions, prophesies, pagasi, and 'vampyres*,' the story is told like a fairy tale. Things simply happen. The prose is simple, direct, to the point. Perhaps this is my fault for coming to this book directly after delving into Terry Pratchett's Discworld for books and books on end, but despite the objectively terrifying elements of horror in this book, it reads like a children's book. I love it. The reader gets very little insight about the main character, Aeriel's** inner life, in part because she is...not the brightest. I don't mean to be cruel, and I will provide textual evidence, thank you very much. She gets through the hardships of her journey by being kind and meeting kind people, and you don't get to see her inner life because what you see is precisely what you get. She is as simple as the words on the page, and frankly, she's kind of charming for it. Even if you do, on occasion, have to smack the page with your bookmark as she takes four tries to understand the very literal, entirely unpoetic and unmistakable, frankly dead obvious meaning of a prophesy about a pegasus' hoof. Bless her heart.
Directly from the pronunciation guide, verbatim:
*vampyre - VAMM-pire (like "vampire")
**Aeriel - AIR-ee-ell (short a as in "arrogant")
I cannot stress to you how deeply seriously this text seems to take itself, and how absolutely hysterical this becomes when combined with "vamprye (like 'vampire')". You don't say. This is brilliant. What cheese. I love it. You see why I need to document this.