Three Times Lucky, by Sheila Turnage

Mo LoBeau is an average small-town kid, with a far from average family life.  She lives in Tupelo Landing and helps run a small cafe with her caretakers, Miss Lana and the Colonel.  They all live in different wings of their home, which is attached to the cafe.  None of them are related; Mo and the Colonel washed into town on the river on floodwaters following a hurricane, Mo as a newborn and the Colonel as a grown man with no memory of his past.

Mo is content with her life, but her past always piques her imagination.  She's been trying to write her own autobiography, but not knowing where she comes from makes that somewhat difficult.  She keeps a number of notebooks and whenever something's on her mind, she writes letters to her "upstream mother".

Mo's small town is suddenly  jerked out of routine when a police detective comes into town, looking for leads, and then stays to investigate the murder of a town resident, Mr. Jesse.  Mo's best friend Dale was the last person to see Mr. Jesse alive, and because he'd "borrowed" Mr. Jesse's boat, they're both afraid he'll be implicated in the murder.  So they decide to solve the murder before the detective gets on Dale's trail.

This was a fun book, but strangely, I didn't feel that this one was exceptional enough to merit inclusion on the Newbery list.