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Gulf by Shelly Campbell
Amazon US $0.99

Seventeen-year-old David is fading from his world, like a Polaroid picture in reverse. He longs to feel connected to something bigger.

When his brothers discover the new extension at the rental cottage comes with a locked door, David finds the key first. Expecting to claim a bedroom, he opens a dimensional gateway instead, exploring abandoned versions of his world in different timelines, 1960s muscle cars alternating with crumbling cottages.

Except now the dimensional bridge won’t close, and something hungry claws the door at night. David scours for clues to break the bridge, but each trip to the other side makes him fade more on his. Even if he succeeds, he risks severing his connection to his own world, and dying on the wrong side, forgotten.