People can read Mr. Karpyshyn's thoughts on the possibility of a third Darth Bane novel on his starwars.com blog (
http://blogs.starwars.com/DrewK/14)
Q: Any plans for another Bane novel?
A: I'd love to do a third Bane novel. However, there are no official plans yet. Ultimately that decision is made by Lucas and Random House/Del Rey, based on their publishing schedule. From the sales of the first two novels I'm pretty confident they'll want to do another, but for now I just have to cross my fingers and wait.