Responses
If you want spoilers here you go:
The book she finds is just fanfiction. It’s not the real novel. It was written by fan who thought Phil never got “punished” enough for taking Elincia’s place, so they went ahead and wrote the most terrible ending. It was never a premonition of what was going to happen.
If you want more spoilers:
The Elincia that we see now is not the real Elincia but the one who wrote the fanfiction. She transmigrated. She is more selfish, mean, and spoiled than the real Elincia.
Is the book telling MC her future actually her magic manifested? Like she has no real powers but what if she reincarnated (without her past memories), or at the very least her past self sent the book back to her younger self to prevent her terrible future.