The Bluecoats: Sallie (The Bluecoats #16)

A quiet day in the Union Army... Soldiers are resting, Blutch and Chesterfield are arguing, and the generals are plotting strategy. Things change suddenly with the arrival of a new regiment, sent as reinforcements to counter the imminent arrival of the Confederates. With them is a young dog, Sallie, who's been on every battlefield with her uniformed masters, and who takes an immediate liking to Chesterfield, to the point that she accompanies him on a dangerous scouting mission...