The Unwanteds #4: Island of Legends by Lisa McMann

Alex and the rest of Artime are still recovering from the death of Mr. Today and the rescue of Sky and Crow from Warbler Island. However, Queen Eagala leads an attack to recover the escaped prisoners. Later, Alex leads a group to rescue Sky's mother from another island. Florence is captured by a giant eel in the process, so the force must save her along with several other creatures. This adventure then leads the group to the Island of Legends where they encounter creatures living atop the shell of a huge crab. Meanwhile, Aaron, Alex's twin brother, is trying to prepare the citizens of Quill for an attack on Artime. He's also discovered a secret jungle where Mr. Today was hiding some dangerous, uncontrollable creatures. Aaron believes he can use them as secret weapons against Artime

You should read the whole series in order, since each book continues where the previous book left off. Type Unwanteds in the search box at the top right corner to see reviews of all four books. You'll need to accept some strange characters, because there's a talking statue, rock, fox, and Alex even creates a talking whale. There are a couple of plots going on, and they will lead you to several different settings. This book doesn't have as much action as the first three books, but it's still entertaining. If you read and enjoy the first book, The Unwanteds, then you'll like this book too.