This is book number 110 in the The Boxcar Children Mysteries series.
The Aldens are preparing for a yard sale when they find an old wooden mask in the trash behind a neighbor’s house. Violet notices that the mask looks like a Native American mask she had read about. When the mask is stolen, the children begin to think their newfound treasure may be worth a lot to someone. Can the Aldens find the thief and the mask before it’s too late?