Is it possible that you've read the book before and forgot? Just asking if that's possible.
Otherwise… In your shoes, as long as I was 100% certain I'd never read the book before, I'd leave it, look for the differences between the works, and contact the other book's author to give her a heads-up along the lines of how surprised you are at the similarities between your books, great minds think alike, and with the differences X, Y, and Z…would she be interested in a cross-promotion arrangement?
If I freaked out over similarities, there's one author I wouldn't be able to enjoy, because her books already have major plot elements and even made-up names that I've used in my writing before reading her books. Even my writing style was similar to hers, when I first read her, where there were word-for-word sentences and creative phrasings that matched the first book I'd read by her, in things that I'd written well before I'd read the author's work.
We all have people who think along the same lines we do, and life and local and world events will trigger some same ideas for people. My Chronicles of Marsdenfel is necessarily unusual-in part because I started it by looking at a trope list and figuring out how to either use them legitimately or thumb my nose at them. I've still not found another author who embodies all that I am aiming for with that series, but I've found things like, "Oh, this is comparable in X manner, but not Y."
One of the few comparable-in-type-of-fantasy-world-and-overall-tone authors I've found finished her series and announced her next project…with pretty much the same tagline I was using for a story world I'd already created, though I'd not released anything in that world. I only had a short story finished, so I released that, and I still haven't been able to get back to that world, because I have to finish some other stories first, but at least I have that published before hers.
So my advice is to take a deep breath and look for the differences.
Technically, plagiarism is word-for-word copying.
Paraphrases can be plagiarism, too, like if you take another person's book and rewrite it scene-by-scene.