Picked up Wait Till Helen Comes at the farmer's market. Hahn takes her talent for combining kid's family problems like lost secrets and general friction, then combines them with more tension in the form of a ghost story. Her books may be "simple" chapter books, but the story is scary enough to raise chills in an adult. [Guilty.]
In this 1986 book, Hahn tells the story a blended family -- the mother with two older kids who are expected to take care of their step-dad's younger daughter who hides a heavy secret. All the kids are unhappy about moving to a new home, but the youngest feels picked on until she finds a "friend", Helen, who wants to take her to a hidden land where she'll be happy forever. Molly, the older step-sister, soon finds herself trying to "save" a step-sister who doesn't want to be "saved". Molly finds herself caught getting blamed for causing trouble because every time she tries to help, her step-sister twists events so Molly appears in the wrong.
One of the wonders of a good chapter book is that it has all the elements of a larger novel, but there isn't as much internal dialog, complicated settings, or description to get in the way of the core. An adult reader can easily follow the basic craft structure of a novel in Helen: the introduction of characters, the complication that emphasizes the danger a character's in, and the working out of a realistic the solution. On this simplified framework, Hahn creates a scary story with a warm and happy ending, a must for younger readers.