I'd love an ID for this, been going around my head for months.

I read this a couple of years ago, could be older, but felt like a modern book

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I read a book where a brother & sister (2 people anyway) got transported to a frenstein type world

They landed in a marshlands with scrub, hid from some monsters, made it to a walled town.

There was a mad scientist lab out of town in a windmill, lightning in a bottle, used lightning (maybe something else) to bring someone back alive.

There was a beach briefly but had a really powerful monster/god in the water

near the end they or one of them went back to our world

Very much Narnia but frankenstein