Titanic, Book Three: S.O.S. by Gordon Korman (Published by Scholastic, September 1, 2011)
From bestselling author Gordon Korman, the thrilling conclusion to the adventure aboard the unluckiest ship of all. The Titanic was supposed to be unsinkable - the largest passenger steamship in the world, one of the biggest and most luxurious ships ever to operate. For Paddy, Sophie, Juliana, and Alfie, the Titanic is full of mysteries - whether they're to be found in the opulent first-class cabins and promenade decks or the shadows in the underbelly of the ship. Secrets and plans are about to be revealed - only now disaster looms, and time is running out. The four of them need to find the truth, unmask the killer...and try not to go down with the ship.
After reading the first two books in this trilogy, I am dying to read the final book and find out what happens to all of the characters when the Titanic sinks. I almost wish now that I had waited until the whole series was published and read them all together!