The Big Six by Arthur Ransome: Book #19 for 2017
This is effectively a sequel to the fifth Swallows and Amazons book, "Coot Club", which - like this one - was set in Norfolk and did not feature the Swallows or the Amazons. In fact, inn this title, they don't get mentioned once.
The story starts out with Tom Dudgeon, introduced in "Coot Club", and his friends being accused of unmooring several boats, based on the fact that it all started when they arrived in the area. As you can probably guess, they have nothing to do with any of this, but things get worse when a pair of stolen shackles get planted to make them look like the culprits.
Eventually, Dick and Dorothea Callum, who featured in three of the previous books in the series, show up and the six main characters (the "Big Six" of the title) start a little detective work to find out who was responsible.
While I didn't enjoy this as much as previous titles - characters like Roger and Nancy, who are not in this book, are better written than Dick, Dorothea or Tom, and the plot was less exciting than the last few books in the series - I did like the fact that this book did not feel like any of the previous titles in the series, and so felt like Arthur Ransome was trying something completely original. Most of the second half of the book was about the attempts to catch the elusive culprit, and while their identity wasn't that surprising, it still made for an enjoyable read.