The Mystery of the Pilgrim Trading Post, by Anne Molloy (epub/Kindle/pdf)
Twins Will and Lettie Dennis and their cousin Jonas Wingate are unwilling visitors to the old Maine homestead of their older cousin, Mary Pete Tibbets. She enlists their help in trying to prove the home is the site of a former Pilgrim trading post. Proving the mansion's historic past is the only way to save it from demolition, as the town wants to tear it down and build a new bridge in its place.
"Stowaway adventure, smuggling, possible poisoning, and a salvaged romance are credibly presented by the author who conveys depth of characterization through conversation and the incidental inclusion of Maine's human, herbal, and wildlife. Regard for natural beauty and historic landmarks is an intrinsic value in this mystery which combines a graceful style and absorbing action." -- Kirkus Reviews