“A dazzling plot with characters who are rich and real and unforgettable.”—Lynda Mullaly Hunt, New York Times best-selling author of Fish in a Tree
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.
Book Details
Format:
Hardcover
Price:
19.99 USD / 25.99 CAD
Published: 07/14/2026
ISBN: 9781635923261
Imprint:
Calkins Creek
Page Count: 368
Trim Size: 5-1/2 x 8-1/4
Grades:
Grade 5 & Up
Ages:
10 and up
“A dazzling plot with characters who are rich and real and unforgettable.” —Lynda Mullaly Hunt, New York Times best-selling author of Fish in a Tree
“Barr’s Thunder and Mercy brilliantly draws young readers into the past through a clever and relatable main character whose voice feels fresh and alive. What makes it stand out is the way it balances that energy with meticulous attention to historical detail—small, surprising touches that deepen the story and bring its world vividly to life. It’s both engaging and richly grounded, a combination that will spark curiosity and open new doors to history for young audiences.”—Justina Barrett, chief learning and engagement officer, Historical Society of Pennsylvania, and former site manager for Mount Pleasant