This is the audiobook of the Spy Series by Selah Amundson. Selah is 10 years old, and this book is completely unedited by any adult. Three spies come home to find the following letter:

"Stella, 
Feveral has come with Lenny and about fifteen other Assistants and captured everyone. You, Kate &amp; Skyler, are the only people left. I need your help. I don’t know where I’m going. All they would tell me is that we’ll be very near water, but Leana told me we’re going to Rhode Island. I know it’s not enough information but if you see Leana. I’m sure she’ll tell you more.
Good luck.
- Mr. Coranam"