As a librarian, my job is to get books into the hands of readers who would appreciate them. Monte Cook has been good enough to mention some of his inspirations (not the least of which is Gene Wolfe's New Urth saga). Continuing in this vein, I figured I'd give a few recommendations for reading to inspire the Numenera at your gaming table.
Frank Herbert, Dune
This is a no-brainer. The action that happens on-planet, the contrast between low and high tech, the esoteric fighting styles of the Bene Gesserit and their weirding way, the breeding program that produces the Kwisatz Haderach--all juicy little morsels that can spark your own creativity.
In the third podcast of Transmissions from the Ninth World, Monte Cook mentions the works of French comics legend Moebius as an inspiration for Numenera, and I cannot agree more. In fact, Moebius's stories, his plots and style, are also excellent sources of inspiration. In many of Moebius's science fiction tales, exploration and a sense of wonder are paramount. His sense of the cosmic existing side-by-side with the mundane sets a tone for many a potential Numenera adventure.
Kay Kenyon, The Entire and the Rose series
Kenyon posits a universe created by otherdimensional beings that parallels and resonates with the beauty and bizarrity of the Ninth World: a bureaucracy patterned after the Chinese Empire; powerful sentient beasts who form telepathic bonds with their riders; a host of unusual races gathered together by the world's creators; sailing submersible ships that move through quantum streams; storm walls that soar to the sky, holding back an encroaching chaos and giving form to the world. The first book in the series is Bright of the Sky, and trust me--you'll love this.
Edgar Rice Burroughs, A Princess of Mars
Bizarre alien races? Check. A whole world of ancient tech created by previous dynasties? Check. A protagonist with incredible powers? Check. (Think about it: John Carter as a Glaive.) Daring adventure, swashbuckling, savagery, and romance? Yup! How can you NOT read this book and think of Numenera?!
Larry Niven, Ringworld
A massive construct, home to scads of alien races, created by unknown beings (now revered as gods by some of the natives) for an unknown purpose, just waiting to be explored, with lovely bits of bizarre tech to help them along. I think you can mine this book alone for an entire campaign! (And who's to say that your Numenera *isn't* a ringworld?)
So there's a bit of reading to pique your interest! Hopefully, you'll find these works as inspiring as I did, and pass on the recommendations yourself! And if you have any more to add, please do!
P.S.: I find the entire subgenre of planetary romance (a.k.a. sword and planet) to be perfect fodder for Numenera inspiration--Google the terms and see what you find!