Harriet Spies by Elana K. Arnold

From Walden Pond Press:

The unforgettable star of Just Harriet returns for another mystery on Marble Island, from award-winning author Elana K. Arnold.

There are a few things you should know about Harriet Wermer:

• She always tells the truth.
• She’s loving spending her summer on Marble Island, where she is an A+ mystery-solver.
• Okay, maybe she doesn’t always tell the truth.
• Actually…she has a tendency to lie quite a bit.

Which is why, when one of the guests at her grandmother’s bed-and-breakfast finds that their treasured pair of binoculars has gone missing, no one believes Harriet when she said she had nothing to do with it. But this is one time Harriet isn’t lying—and she knows that if she can find the binoculars and figure out who really took them, she can prove it. 

With her cat, Matzo Ball, her grandmother’s basset hound, Moneypenny, and Harriet’s new friend, Clarence, helping her out, Harriet knows she can crack the case. But when the culprit isn’t who Harriet expects, it’s up to her to decide how important the truth really is.