The Unseen Guest by Maryrose Wood
Why did I think The Incorrigible Children would only be a trilogy? Wishful thinking? The pacing is so tedious that I want it to end, but the story is so charming that I keep reading it anyway. I want to see Penelope finally solve the mystery, but it is frustrating me that she is so obtuse.
I do think these would be fun to read aloud to young children, but since my thoughts on this book are essentially the same as the other books in the series, I will just refer you to those reviews.
The Hidden Gallery
The Mysterious Howling