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Thunder and Mercy

A Novel

Jennifer Robin Barr

$45

Hardback

Forthcoming
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English
Calkins Creek
12 August 2026
""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.
By:  
Imprint:   Calkins Creek
Country of Publication:   United States
Dimensions:   Height: 210mm,  Width: 140mm, 
Weight:   567g
ISBN:   9781635923261
ISBN 10:   1635923263
Pages:   368
Publication Date:  
Recommended Age:   From 10 years
Audience:   Children/juvenile ,  English as a second language
Format:   Hardback
Publisher's Status:   Forthcoming

Jennifer Robin Barr grew up in the Philadelphia area during the 1970s and '80s, spending her summers immersed in imaginative play with neighborhood friends, whether diving into games like home free or acting out scenes from Charlie's Angels. Her writing focuses on weaving fictional stories around historical events, especially those lesser-known stories from Philadelphia's past. She finds inspiration in the city's historic neighborhoods and old cemeteries. Jennifer's debut novel, Goodbye, Mr. Spalding, was named a 2019 Booklist Top 10 Sports Book for Youth. Visit her at jenniferrobinbarr.com.

Reviews for Thunder and Mercy: A Novel

★ ""Set in 1979, the novel captures the freewheeling feel of a Philadelphia summer while weaving in high-stakes Revolutionary War espionage. Thunder is prickly but compelling, determined to prove herself apart from her science-obsessed parents by solving a centuries-old puzzle. Charlie, an earnest eight-year-old with meteorological aspirations and a permanent tie, and his sweet but struggling sister Heather, round out this trio of latchkey sleuths who will inspire readers to form puzzle-solving crews of their own. Barr moves confidently between eras, blending historical fiction with brisk, thrillerlike momentum. The result is an immersive adventure filled with coded messages, spy lore, and the ache of growing up with largely absent parents... A must-purchase for middle grade collections. A lively, genre-blending mystery that will appeal to readers who enjoy historical intrigue, clever puzzles, and a strong-willed heroine.""—School Library Journal, starred review “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


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