1. Bone. Stick the word "bone" in your title and people will pick that baby off the shelf.
The word is fluid enough to suggest darkness when it may mean simply "the essence, core" of a matter, or something that is used to mollify or placate, as in "throw him a bone".
2. Glass. Glass is solid but old glass shifts and ripples with time. (New glass might shift in years to come. We won't be here to see it.) Glass is transparent. It is fragile. It is reflective. The word "glass" is used to indicate any barrier that is not easily seen. Bingo! ("Crystal" can work, too.)
3. Colors. I mean the names of actual colors - not the word "colors". The book may be about lions but if you add "golden" to the title, you have an attention grabber. "Blue", "green", "red", - oh, and this is a winner - "amber" all give your title an edge. "Amber" is much better than "yellow" or "orange", by the way.
4. Moon. This is a perennial favorite. Anything about the night sky will make readers take a second look, but "moon" is top of the list.
5. Directional words. I mean the class of words that indicate direction, - "into", "on", "through", "over", "under". These prepositions give a sense of depth to the title, a sense of things not easily seen. The book will take the reader somewhere else and reveal details that only the author can impart.
So, how about this title? "Bones Under the Amber Moon".... oh, I want to read THAT book. Oh wait, I skipped "glass".
How about, "A Bone Moon in a Purple Glass" ?
Write me some books. Carry on.