Yes, Lord Sunday is the last book in the series. It was released in early 2010.
The series has dealt with Arthur starting on a Monday all the main antagonists for each book have been named after a certain day of the week -- Monday through Saturday have already been done (except the cliff hanger ending-- did Arthur get the key before Saturday zapped him will he survive his fall etc.) and due to the fact that all parts of the house are now in Arthurs possession (except the gardens) it is safe to assume that Sunday being the last day of the week and the only trustee left to remove a key from is the last book.
every book up to superior Saturday is out and lord Sunday is out IN spring 2010 (i think)
As far as i know Lord Sunday will be released early spring 2010
In the US. it will come out on March 4th