Pay Dirt Road by Samantha Jayne Allen
Annie is back home after college, waitressing at the local diner and hanging out with her cousin and people she knew in high school. Her family is well known in town. Her grandfather used to be the sheriff, but now he owns a private investigation firm, run mostly by his partner, and drinks too much. When another waitress, Victoria, goes missing and is later found murdered, Annie is pulled in, needing to find the truth, and gets her grandfather involved too.
Pay Dirt Road has a good sense of place. It's small-town Texas where land matters, where it's hard to keep secrets, where people without papers are afraid of the cops. It's a place where high school football carries a town's pride and the VFW turns into a honky tonk on Thursdays. It's a place Annie both loves and hates.
Pay Dirt Road is a pretty standard mystery and Annie's not a great investigator. She shouldn't be. She's in her early twenties...