Thunder and Mercy: A Novel

Winner of the 2026 Grateful American Book Prize

“Intrigue and espionage abound in 'Thunder and Mercy', a gripping historical novel by Jennifer Robin Barr set in 1970s Philadelphia... 'Thunder and Mercy' proves its point: Uncovering the past can be a thrilling way of playing detective.”— The Wall Street Journal

Is a hidden diary the key to an unsolved secret from the American Revolution?

It’s 1979 and twelve-year-old Theodora “Thunder” Underwood is dreading her summer vacation once again. Every year, her scientist parents disappear on far-flung scientific expeditions, leaving her with relatives who clearly don’t want her around.

But this summer, Thunder is sent to Mount Pleasant, her aunt’s historic yet crumbling old mansion outside of Philadelphia once owned by Revolutionary War traitor Benedict Arnold. When Thunder discovers a mysterious diary written by Mercy, a girl her age who lived through the American Revolution, Thunder is plunged into a mystery that involves a shadowy prowler, a hidden ring, and mysterious spies—making her summer far more exciting than she ever expected.

Fast-paced and full of twists, this suspenseful middle grade novel transports young readers between the 1770s and 1970s as Thunder uncovers the truth behind a Revolutionary-era spy network—and discovers the importance of friendship, family, and her own voice along the way.