There was a number of reasons this book was chosen as the follow up. First, because this book contains the Pevensie children (Lucy, Edmond, Susan & Peter) and so if the series was to have the same actors they needed to make the film versions with these actors as soon as possible.
Secondly, the Prince Caspian story is less controversial than the book "A Horse and his Boy" which traditionally follows the Lion, The Witch, and The Wardrobe. No, it's not controversial for the same reason's of Equus, it is controversial because of the potential of being interpreted as an anti-Muslim film.
Andrew Adamson, director of the first Narnia film as well as the first two Shrek films, has recently begun releasing names of people who will join the cast.
The most important name for this second release is Ben Barnes (right), who has been announced as the new Prince Caspian. Barnes who is 25, will actually be playing the part of a 13 year old in this first film. Barnes, who has recently been "playing young" will be leaving his role as a character in the London stage version of The History Boys, he also appears in this years fantasy film Stardust.
Also of interest, Peter Dinklage (of one of my favorite films, The Station Agent, pictured left) has been cast in this film playing the role of Trumpkin the Dwarf.
Other cast announcemnets include Vincent Grass as doctor Cornelius and Spanish actress Alicia Borrachero as Prunaprismia, the wife of Narnia's evil king.
ALSO...Andrew Adamson appears will not be directing the entire franchise, as filming of the film The Chronicles of Narnia: The Voyage of the Dawn Treader will begin filming before Prince Caspian is in the can. (source)
Related Tags: Prince Caspian, The Chronicles of Narnia, Stardust, Ben Barnes, Andrew Adamson, Vincent Grass, Alicia Borrachero, Voyage of the Dawn Treader, Peter Dinklage, Lion the Witch and the Wardrobe, Fantasy, Upcoming Movies, Film