I finished reading the last of the books in the keys to the kingdom series today and I will say this is probably one of my all time favorite series. I've never read anything by Garth Nix that I didn't like and these books are no exception.
If you want to read the books I highly recommend them and they don't take long to read they are written on the level of a children's book and you can read through one of the books in just a few hours. If you don't care to read the books or have already read them then read on otherwise consider skipping this post until I won't be spoiling the books for you.
The essential plot of the series is that the universe was created by a being called the architect who also created a whole other dimension called "The House" which watches over the universe. The House is divided into 7 realms corresponding to the 7 days of the week and particular denizen of the house is given one of seven keys giving them power over one of the days of the week in the universe and that corresponding domain within the House. After a nice long 15 billion year run the architect decides to call it quits and commit suicide. Apparently the architect is unable to commit suicide herself since she imprisoned a part of herself in a prison which she made to last until the end of her creation. Unable to kill herself without taking everything else with her she breaks herself into 7 parts each one being a part of the "will" of the architect. Which we are meant to take in the sense of "last will and testament". These 7 parts of the "will" are given to the 7 rulers of the days. Each part of the will telling a different ruler to relinquish their power to a mortal "rightful heir". Whereupon all of creation would be destroyed and the "rightful heir" allowed to build a new universe as they saw fit.
There is a bit of a complication in the carrying out of the will since those entrusted to carry it out decide not to commit suicide and destroy the universe. Instead they lock away the different parts of the will for 10,000 years at which point the story line of the books actually begins when one part of the will escapes and manages to dub a mortal rightful heir. The rightful heir then goes on to slowly gain power and dominance in the house collecting each of the keys and each part of the will in turn. All the while blissfully unaware of the fact that the ultimate goal of the will of the architect is the destruction of everything. Our hero Arthur, the rightful heir, works over the course of all 7 books in an effort to save and protect which he does seem to manage to do at least right up until the last part of the last book when all of creation is expunged.
I have read rather few books where the entire universe gets destroyed. I must say I didn't expect that to happen what with the heroes generally managing to save the day, especially in a children's book. I must say though a 7 book series has got to be the most elaborate suicide story I have ever heard. (though since the true purpose of the will is concealed right up until the last few pages of the last book the books really read just like any other children's adventure story).