I recently read the middle-grade fantasy adventure Deep Blue by Jennifer Donnelly, the first book in the Waterfire Saga. It's about a mermaid world just below the surface of our world, so I'd been curious about it for a while.
It was a fun read that moved very quickly. I thought the characters were engaging, and the author fashioned a rich world with its own Atlantean mythology and history.
I was also drawn in by the cover which was created using a combination of underwater photography and digital effects.
Here's a short video about how they created the cover:
I'm planning to check out the other books in the series, Rogue Wave and Dark Tide soon, too.