The last book Diana was working on before her death, The Islands of Chaldea, has been finished, and will be out in the summer of 2014!!!! It was nearly completed, but had no ending...happily, her sister Ursula was able to finish it.
Here is the Publishers Weekly article with all the information.
Ursula Jones who wrote one of the books that has been sitting on my very own TBR pile for almost a year, The Lost King, was Diana's sister. Guess what I'm reading this weekend...
(which would be Ursula's book. Not Diana's. Not yet....)