The Parker inheritance - Varian Johnson

When Candice finds a letter in an old attic in Lambert, South Carolina,
she isn't sure she should read it. It's addressed to her grandmother,
who left the town in shame. But the letter describes
a young woman. An injustice that happened decades ago. A mystery
enfolding its writer. And the fortune that awaits the person who
solves the puzzle.
So with the help of Brandon, the quiet boy across the street, she
begins to decipher the clues. The challenge will lead them deep
into Lambert's history, full of ugly deeds, forgotten heroes, and
one great love; and deeper into their own families, with their own
unspoken secrets. Can they find the fortune and fulfill the letter's
promise before the answers slip into the past yet again?