The fortress of the treasure queen

With nearly 2 million books in print, this Little Apple series is H-O-T, hot. The SECRET is out - DROON is the series that kids, parents, and teachers are talking about!Ko is back in power -- and nothing in Droon is safe! His beasts have found a lightning-powered submarine hidden in the secret Fortress of the Treasure Queen. Now Eric and his friends have to stop Ko before he destroys Jaffa City. But first they'll need help from their worst enemy -- Lord Sparr. If he can be trusted . . . .