So this time, Jack and Annie visit the magic tree house and open up a book on the Civil War. I now understand that there is, threaded through the books, a story about Morgan LeFay, the magical librarian of Camelot, and Jack & Annie help her in various ways through their time traveling. Now, she needs them to collect different types of writing, and the first time is "Something to follow."
Fortunately, Jack & Annie wind up at a field hospital run by none other than Clara Barton, and one of the nurses gives them a list of instructions re: how to be a nurse. The kids assist the nurses by delivering food to the injured men, following those instructions (#1: Be cheerful) as they do so. They learn about the Civil War and Clara Barton as they work, and they also get an emotional understanding of the devastation war causes.
This was better than the first book. I'm having trouble with the magical librarian bit, but that wasn't a huge part of the book, so. I appreciated the emotions Jack & Annie experienced at seeing how young some of the soldiers were, and the one scene where the kids were in danger (flying mortars) felt much more immediate.