Mystery of the Fires (version 2), The by Edith Lavell (1892 - 1957)

Mary Louise Gay decided to spend the summer holiday in her family's cottage at Shady Nook. But an unknown arsonist has been setting the cottages on fire and it's up to Mary Lou, her best friend Jane, and their loyal pup Silky, to find out who the culprit is and save the houses and their summer holiday. - Summary by Mary Escano