The book you're not supposed to have is the fifth book in the Timmy Failure series. It was on September 27th, 2016. It is the first book in the series to feature anyone other than Timmy on the cover.
Timmy's behavior has never been willfully destructive, but in this case, he might be accused of trying to sabotage his mother's wedding, or at least simply continuing to use detection as a means to impose his will on an often confounding world.
- Timmy's mom gets married in this book.
- First appearance of Toots, who later becomes the superintendent.