Books in the Secret Zoo series

Something strange is happening at the Clarksville City Zoo. Late at night, monkeys are scaling the walls and searching the neighborhoodbut what are they looking for?Noah, his sister Megan, and their best friends, Richie and Ella, live next door to the zoo. Megan is the first to notice the puzzling behavior of some of the animals. One day Megan disappears, and her brother and their friends realize it's up to them to find her. Their only choice is to follow a series of clues and sneak into the zoo. But once inside, will they discover there's much more to the Clarksville City Zoo than they could ever have guessed?