Percy Jackson: The Sea of Monsters will be released on March 27, 2013.
It is still not confirmed whether or not Chris Columbus will direct the film. More info should be revealed soon.
And this is a short summary of the second book:
When Thalia’s tree is mysteriously poisoned, the magical borders of Camp Half-Blood begin to fail. Now Percy and his friends have just days to find the only magical item powerful enough to save the camp before monsters overrun it. The catch: they must sail into the Sea of Monsters to find it. Along the way, Percy must stage a daring rescue operation to save his old friend Grover. He also learns a terrible secret about his own family, which makes him question whether being the son of Poseidon is an honor or a curse.