Long Lost Elvis by Christine Isley-Farmer

A donkey runs out in front of Nana Weathers's car when she, Chloe, and Boomer are returning from Camp Resound. Mr. Brown, a farmer, drives upon the scene. To thank them for keeping his donkey, Elvis, safe, Mr. Brown invites everyone to his farm. There, they discover that his wife's favorite painting is missing, and so begins a new mystery for Chloe and friends to solve. Will their efforts be successful, and what surprises will they learn about Elvis, the runaway donkey?