Mary Pope Osborne - Civil War on Sunday

Book 21 in the Magic Tree House series (2000)
A novel by Mary Pope Osborne
Traveling back in time from the rumbling thunderstorms of present-day Frog Creek, Pennsylvania, to the booming of Civil War cannonballs near Richmond, Virginia, Jack and Annie set out on their 21st Magic Tree House adventure. The mysterious Morgan le Fay, magical librarian of Camelot, the long-ago kingdom of King Arthur, has left the brother and sister a message in their magic tree house, asking for their help saving Camelot. "Please find these four special kinds of writing for my library: Something to follow, Something to send, Something to learn, Something to lend." Jack and Annie enthusiastically transport themselves to a field near the fighting, and soon are enlisted as volunteer nurses assisting none other than Clara Barton, legendary "Angel of the Battlefield," as she drives her horse-drawn ambulance right onto the battlefields to help save wounded soldiers--including one with a very special connection to Jack and Annie.