The cowgirl aunt of Harriet Bean - Alexander McCall Smith

Harriet's invited to accompany her aunts Thessalonica and Japonica to America. She's terribly excited, knowing very well that with two expert detectives for aunts, this is going to be no ordinary trip.

She soon discovers, to her enormous surprise, that they're actually on their way to see her sixth aunt, Formica! And as if this weren't surprising enough, she finds out that Aunt Formica is a cowgirl, and the reason for their visit is to help her catch some rustlers that have been stealing cattle from her ranch.

Harriet and her three aunts must move like greased lightening if they're to get on the trail of the slippery cattle thieves. It's time to saddle-up and yee-ha!