This is the first book in the Keys to the Kingdom series by Garth Nix. It was supposed to have a book every six months for 2 1/2 years, but it seems Mr, Nix unfortunately could not deliver on that so it is like most other series, in 1 every year or so(give or take a month). Which is kind of annoying if you wanted them every 6 instead of 12 months, but the quality of writing would be hurt I believe if that had occured so I am quite happy waiting for the book "Sir Thursday" right now. Mister Monday takes place on a Monday(make sense) when Arthur our "Hero" happens to become the heir to a will that was divided a century or so ago by the "Morrow Days," which are the days of the week,a nd they operate in the "Secondary Realms" only on the day in which they are named. This was not supposed to happen, the Days were supposed to keep the Will whole and pass it on to a rightful heir without conflict. Which of course can NOT happen or we would not have this great series. So Arthur has asthma VERY severely in fact and almost dies, but survive because Mr. Monday gives Arthur the minute hand to the "Key to the Kingdom" that he posesses. The key makes it possible for Arthur to live through a brutal asthma attack on the first day at this new school. Now Arthur starts to see things that he had never seen before, and must make it into Monday's postern to get to the house, grab the Hour hand to complete the key and take over 1/7 th of the houses power. So along the way he meets Suzy Turquoise Blue who once was a human like Arthur, but has become a "denizen" of the house where it is almost impossible to be killed/die and you never Have to eat or drink, and never get colds. They are used a fashion accesories and show the other denizens you have a status as they are hard to acquire and expensive. So Arthur and Suzy have many encounters and then are captured, and Arthur is thrown into the Coal Cellar of the realms and meets the  "Ancient One" who tells him the stories and some of the secrets of the realms inside the house and about the "improbable stair" which only excists if you are able to see it and able to transfer from one realm to another quickly without losing it and being stranded in the realm forever. Eventually it leads to a battle which is quite large in scale and Arthur does triumph even with his asthma and various other faults, which makes him a likable character, he is not perfect, He is a child and has health problems, so he is more real than these characters who have no faults at all. This series is great, and I recommend it to anyone to read it, enjoy it and read the other 2(so far) in the series, as well as his Seventh Tower, and the Sabriel, Lirael, Abhorsen series, theya re all great
This is the first book in the Keys to the Kingdom series by Garth Nix