Flowery, Fragmented
Most of this novel is written from either the perspective of an incredibly intelligent teenage psychotic or the perspective of a highly educated, world-renowned psychotherapist. When the main characters in your book are serious brainiacs, your writing style is going to be chock full of SAT words.
Yeah, the language can get a little thick with description, historical and literary references, and Deborah's own Yri language. Not all of the sentences are long and winding, but you might need a dictionary here and there.
Flowery
Many of the flowery descriptions in this novel are coupled with Deborah's Yri language. When Deborah makes friends at summer camp with a girl named Eugenia, for example, Deborah worries that her poisonous essence, which she terms "nganon" in Yri, might have infected Eugenia because Eugenia starts exhibiting some crazy behavior—like whipping herself with a leather belt and asking Deborah to do it to her, as well.
Deborah goes on: "Then it suddenly came to her that Eugenia's nganon might be more virulent than hers. Even so, to witness was to share; to share was to be responsible. Her nganon had called to Eugenia's…" (17.69). There's a kind of flowery poetic quality to the rhythm of a lot of the writing when we're in Deborah's point of view. She's a creative girl who's built the world of Yr for herself, so she sees the world in very poetic terms.
Symbols, gods, prophecies—Deborah is all about the flowery.
Fragmented
The flow of the writing in the novel often feels fragmented, since we hop between the settings of Earth and Yr often. We also hop around among different narrative perspectives, given the third-person omniscient point of view. To top everything off, we also hop around in time.
During Deborah's sessions with Dr. Fried, we shift between the present at the hospital and all the events of Deborah's childhood that landed her there. There are many physical breaks in the text that represent moments when Deborah leaves Earth for the Pit of Yr—and then comes back to Earth.
There's often lost time between this world-hopping. For example, at one point, Deborah feels the punishment from gods of Yr coming on. Her gods punish her for telling the secrets of the imaginary world. When Deborah retreats into Yr and is punished, there's an actual space on the page in the book. Then the next paragraph starts with her in a cold-sheet pack.
These meandering kinds of descriptions can be disorienting. But that's the point: those sections of the story are trying to show us what Deborah's going through.