I recently had the chance to review Charlotte and the Mysterious Vanishing Place, book 2 in the Kare Kids series by Charles Salter.
This book concept was really appealing to my daughter - she's a nine-year-old girl herself who loves to dream up adventures, so reading about other young girls being heroes is great for her. In the book, Charlotte discovers a lost puppy on the same day that she sees part of the forest vanishing. After rescuing the puppy, she goes back and notices the sinkhole growing. Although she's told by adults that she should stay away, she's worried about other puppies getting trapped and needs to save them.
The book has a good level of suspense, and a predictably good ending, appropriate for middle grade readers with a sense of adventure.