Keys to the Kingdom is a series of books by Garth Nix, covering a single convoluted story arc across seven books. It is nominally a young adult series, and is in the fantasy genre, although like much of what Nix writes, it is not stereotypical high fantasy. The first book was released in 2003, and the last one was released in 2010, making the series a magnum opus of sorts.
Having read Garth Nix's books for many years, and never having been disappointed, I think that just about everything that Garth Nix writes will end up in the canon of young adult fantasy literature. So whatever I say about these books is in comparison to the pillars of the genre, not just comparing them to run-of-the-mill young adult novels.
The plot of the books involves one Arthur Penhaligon, a somewhat shy preteen raised by a retired rock star and a world famous epidemiologist who is visited during a near-fatal asthma attack by one Mister Monday, who bequeathes him a magical key, which he then removes, because he is just trying to circumvent the "Will" of the Architect, a mysterious demiurge-like figure who has apparently left both the House, a sprawling supernatural realm divided into seven sections, each ruled by a supernatural being named after a day of the week. Meanwhile, back on earth, a series of plagues leads to tactical nuclear weapons being used to biologically disinfect areas of outbreak. Meanwhile, the "Will" of the architect, which gradually forms into a commanding, Machiavellian women through the absorption of several animal-shaped avatars, seems to have an agenda of her own, one that may be opposed by the Piper and the Mariner.
If the description of the plot doesn't make sense, don't feel bad: the book is even richer in details, secondary characters, strange places, plot twists, schemes and counter-schemes and a general kitchen sink of ideas than can be explained briefly. The work does stretch to over 2000 pages in all, and Garth Nix manages to throw in a lot in those pages. All of his fantasy to date has been imaginative, and in this series, his imagination broke free, and the wildly shifting plot and scenery has a hallucinogenic quality to it. More specifically, these books are probably the best literary equivalent of disassociative use I have ever seen. And all of it is sometimes hard to keep track of.
In some ways, the books are a great success, because I have rarely seen so much imagination in a fantasy book. Fantasy, somewhat paradoxically, manages to be fairly predictable. But the books also have a weak point: they introduce so many characters, themes, locations, references, plots, objects, etc, that when they actually reach their somewhat-surprising conclusion at the end of Book Seven, their seems to be some loose ends. In other words, they lack some of the unity that a reader could expect, and certainly they lack the unity of The Old Kingdom Trilogy. However, "too creative" is hardly the worst thing to be said about a literary creation, and that is the sum of the books' flaws.