The Seventh Book of Shadows (Fantasy, YA)

Really interesting story concept!! I’ll paste excerpts from your query and comment underneath using [CDH - ]
[CDH – I got a little lost in the 1st paragraph, but LOOVE the part about having only ghosts and shadows for company – it let me know what her life there is like. I’d need to know why she doesn’t think much about her powers, or does her nose twitch with each spell, does she sigh in boredom as she goes through her lessons? I want a feel for her.
Option to rearrange it just a tad with a bit more flare:
Cassandra is a 16-year old witch who’s sheltered/smothered by her Grandmother’s “protection”/O.C.D. on a hidden island with only ghosts and shadows for company. She dreams of leaving her family’s stupid legacy, and living with the humans, falling in love, and having gossip-girl friendships.
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Everything changes when her Grandmother reveals the source of their power, and tells Cassandra that she must either renounce her birthright and leave the island, and her protection, or find a way to bind the adult magic that has started growing in her.
[CDH – Okay, I’m super stupid and prob should have read the whole thing. I think you can combine the 1st and 2nd paragraph and then have more words to give a deeper sense of Cassandra and the stakes she’s face with.
Option to combine them:
Smothered by her Grandmother’s “protection” while living on a hidden island with only ghosts and shadows for company, sixteen-year-old witch CASSANDRA conspires to leave her family’s stupid legacy, and venture into the human-world, to love and experience real friendships/live a mortal life.
=THIS SHOWS HER CURRENT-STATE ISSUES AND HER GOALS.
But the itching under her skin that has waylaid those dreams makes sense when her Grandmother reveals the source of their power. Now, Cassandra struggles with worse-upon-worse options: renounce the power of her birthright and leave the island forever, including her Grandmother’s protection, or determine how to bind the adult magic manifesting within her.
If she fails, she will die, burned from the inside out.

If she does nothing, she could die, consumed by the magic.

If she leaves the protection of the island, she’ll be killed because of who she is.
=THIS SHOWS THE OBSTACLE TO HER GOALS. Question: Does she have to bind this magic to her, or to a crystal, or to the island itself? I think it would be a great idea to make the distinction. AND, THE LAST 3 SENTENCES ARE REAL CONFLICTS (AND GOOD ONES 😊 )
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There are no easy answers but she must choose, because safety is an illusion when everyone wants you for your power. Suddenly the loneliest witch is surrounded by people; a beautiful girl who wants to take her away; long-lost relatives all trying to help her; a man cloaked in darkness and smoke who feels like power.
[CDH - ‘WHY’ do they want her power – do they want to kill her to take her power, or just gain her allegiance for their own evil plans?
Are the long lost relative really trying to help her, or are they ‘claiming’ to want to help her – I know it seems menial, but its word plan that helps us understand the stakes.
This man cloaked in darkness – what does this do to her, is she tempted by him or frightened of him? Does his darkness sing/lure her to him? Give me some details so I know what’s she’s struggling with.
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After finding an ancient book of demon lore called “The Seventh Book of Shadows”, Cassandra starts to suspect that it’s not just witches who want to kill her.
[CDH – don’t worry about saying ‘starts to’ – just say Cassandra suspects it’s <what> that wants to kill her now, too.
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Attacked by strangers, relatives, and possibly her own girlfriend, Cassandra decides to fight for what she loves, finding that she loves more than she ever knew.

[CDH – LOVE the last line!! I’m confused why you use the phrase ‘possibly’ her own girlfriend. She does know who her girlfriend is, right? If there’s something more sinister or threatening, please let us know so we get the full weight of her disasters and struggles.
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The Seventh Book of Shadows is complete at 76,000 words, and is the first in a planned series. Thank you for your time and consideration.
[CDH – Remove comma.]

Overall Comments:
Oh, I’m really interested! You have very few things to firm-up to knock this out of the universe! Great job 😊