The Hidden Gallery by Maryrose Wood
It seems odd to say that a book I read in a couple hours dragged, but The Hidden Gallery did. The story itself, a continuation of The Mysterious Howling, should have been lively enough, but Wood's Lemony Snickett-esque explanations seemed intrusive here, slowing the pace rather than providing comic relief or enhancing the mood of the story. In fact, dragging the story out seems to be the main objective in the series. I would have found it more enjoyable to have the "mysteries" introduced in The Mysterious Howling resolved earlier in the book, and new, more convoluted ones introduced, but, no, everything is dragged out as long as possible. Penelope may have pluck, but she seems a bit slow on the uptake in this installment.
Having said that, I think the story of the mysterious wolf-like children and their governess is an appealing one, and I think many children would enjoy the series, especially young but strong readers. The clues are pronounced enough that a younger reader would feel good about noticing them before the heroine, and offer just the amount of mystery to engage their imagination without making outcomes too obvious.