You'd never get a mystery like this written today, I think - the kids are sent away to do up a library, and they live by themselves for a week in an empty house while they do it. The "about the author" section at the back of the book notes that this series was popular with kids because of the frequent lack of adult supervision, and this book's living up to that. I don't think my parents would ever have allowed this, back when I was a kid, but me and my sister and our friends would take our bikes out for the day and spend it at the river, swimming, when we were about the same age as Jessie and Henry, so it's not entirely foreign. It's nice to read about, though, and indicative of more trusting times, I think.

Anyway, the kids are doing up a rundown library in order to save it, which in practice means they spend the week dusting and shelving books, which is not greatly exciting, but the library has a sword passed down from the time of the civil war, and a local thief is after it. They also make friends with a young boy whose father has recently been lost at sea. It's obvious a happy ending is coming to that particular storyline, and it does, but mostly the whole thing is a sort of extended camping trip with books: pleasant, undemanding fun.