Wrapped in a Riddle by Sharon E. Heisel, 1993.
Miranda is living with her grandmother in Oregon while her scientist parents are away studying algae in Antarctica. Her grandmother owns a bed and breakfast called The Jumping Frog, which has a Mark Twain theme. At first, all Miranda is concerned about is finding some friends and fitting in at her new school, but soon strange things start to happen at the bed and breakfast.
First, someone attacks their housekeeper, hitting her over the head with a bust of Mark Twain. Then, some letters that were written to an ancestor of Miranda’s by Mark Twain himself are stolen.
Although Miranda’s grandmother likes to think the best of everyone, Miranda is convinced that the villain is someone living in the bed and breakfast. With her new friends (the main characters from A Little Magic, another book by the same author) Miranda begins her investigation, determined to find the letters for her grandmother, although it may mean solving a double mystery. There is more that one person in the bed and breakfast with something to hide.
One of the fun things about this story is the riddles that Miranda and her grandmother like to tell each other. Even her friends enjoy riddles, and they help provide Miranda with the inspiration to solve the mystery.