Three Times Lucky by Sheila Turnage
Then a lawman comes to Tupelo Landing to investigate a murder. When it seems like Mo's loved ones might be implicated in the crime, she and Dale decide to use their own detective skills to solve the case. Soon the friends are investigating another murder, a long-ago bank robbery, a kidnapping, and the mystery of the night that Mo washed up on shore. Can the Desperado Detectives uncover the truth, before someone else gets hurt? Find out in this wonderful and engaging novel that's full of small-town heart.