Gertrude, Gumshoe: Murder at Goodwill by Robin Merrill

Gertrude is back! When she finds a murder victim at her local Goodwill, she tries to tell the Sheriff's Department everything she knows. Of course, they don't listen. So of course, she must solve the mystery herself. And the only way she can figure to do this is to go undercover—as the Mattawooptock Goodwill's newest team member. Anyone who knows Gertrude knows this probably isn't the best idea. She'll probably get fired. Or arrested. Or worse.