Harriet Tells the Truth by Elana K. Arnold

From Walden Pond Press:

Harriet sets out to solve a poisoning on Marble Island in the third book in the acclaimed cozy mystery series from award-winning author Elana K. Arnold.

There are some things you should know about Harriet Wermer:

• She used to lie a lot, but not anymore.
• Seriously, she only tells the truth now.
• Even though she hadn’t wanted to come to Marble Island in the first place, now she doesn’t want to leave.

It's the truth. With her mom and new baby brother home from the hospital, it's almost time for Harriet to pack up and head home from Marble Island and all the friends she's made. But Harriet doesn't have time to think about that—not when she discovers that Moneypenny, her Nanu’s adorable basset hound, has been poisoned!

Harriet suspects the culprit is one of the guests staying at Nanu's bed-and-breakfast, and she and her best friend, Clarence, are once again on the case. But when someone else falls ill, Harriet’s going to have to sleuth harder, spy sneakier, and be willing to see that sometimes the truth is more complicated than it seems.