Ex Marks the Spot by Gloria Chao

For her entire life, high school graduate Gemma Sun believed it was just her and her single mother against the world, under the assumption that all her grandparents—and her connection to her Taiwanese heritage—had passed away long ago.

Everything changes when a stranger delivers shocking news: her grandfather only recently passed away, leaving behind the first clue to a globe-spanning treasure hunt that could lead Gemma to her inheritance. To claim it, she must travel to her grandfather's homeland of Taiwan. There's just one catch: the only way she can get there is by teaming up with her ex and biggest high school rival, Xander.