Mystery by moonlight - Carolyn Keene

A MOUNTAIN LAKE HARBORS A HIDDEN PAST!
A charming lakeside cottage should be the perfect vacation spot for Nancy and her friends -- until Bess becomes convinced that they're sharing the place with ghosts. Strange thumps in the attic put them all on edge -- except there is no attic. Even worse, their living neighbors are giving them grief.
A pair of bird-watching photographers sets dangerous traps in the woods, a summer camp director chases Nancy and Ned off his property, and the assistant curator of a nearby Native American museum warns them to stay away from an ancient burial ground. Then a surprising discovery reveals a long-lost mystery -- and a family secret as black as night.