Just in Time, Abraham Lincoln — Patricia Polacco

Michael and Derek don't expect the adventure of a lifetime visiting a Civil War museum with their grandmother. But the mysterious museum keeper invites them to play a game, and before they know it, they're walking through a door straight into a very realistic depiction of 1863. They see the destruction at the battlefield of Antietam, and even meet President Lincoln. Soon, they start to wonder if it's really a game, after all--and suddenly they're racing across Confederate-occupied land to return to their own time before it's too late.