Zelda and Ivy: The Big Picture by Laura McGee Kvasnosky

From chapters.ca: After Zelda, Ivy, and their best friend Eugene watch the new Secret Agent Fox movie (some more bravely than others), they are inspired to do a little detective work themselves. With top- secret agent names chosen and notebook in hand, they're ready to get to the bottom of neighborhood mysteries. But when a rainstorm threatens to ruin the agents' campout, can the three foxy friends come up with an undercover alternative? Sly humor, lively illustrations, and a genuine eye for the everyday dynamics among siblings make this a perfect choice for beginning readers, whether old fans or new.