Superior Saturday is the 6th in a 7 book series. I reviewed the last one here. You can get links to the others there as well.
Arthur, the rightful heir, has collected 5 parts of the will and 5 keys. Since using the keys are slowly turning him into a denizen, he has only kept one key in an attempt to keep himself human. In this book Arthur tries to find the 6th part of the will and 6th key, as he also tries to keep the House from disappearing into nothing.
I cannot tell you if he is successful. Not because of a desire to keep you unspoiled. But because this book is a MAJOR CLIFFHANGER.
This series is just amazing. There's adventure and fantasy and mystery. But there's also a lot of deep stuff for adults. There's the religious framework underlying the series...the 7 deadly sins... and so much more. This would make a great book discussion title. Or series discussion title.