Seriously "High" Wizardry: Diane Duane is out of her mind, and I love her for it

This book is insane.

Thought it was gonna be a kid's book about wizard detectives or something, and it winds up being this Hard Sci-Fi, philosophical existential treatise that takes place beyond the observable universe, uses string theory in 1990, depicts a 10 year old creating an entire new race of beings out of a sentient silicon plate the size of New Jersey and a battle against the force that invented entropy at the beginning of the universe.

SPOILER ALERT: The good guys win by enacting Olber's Paradox.  They STOP THE EXPANSION OF THE UNIVERSE so that every point in the sky becomes a light.  The theory of Olber's is if the universe is infinite yet static (not expanding or contracting) every point in the sky would be as bright as the sun, because there would be infinite stars...

Keep in mind THIS IS A FUCKING KID'S BOOK.

I LOVE THIS SHIT!!!

Also, there's a random character that helps Dairine through the continent-sized airport on Rirath B.  This turns out to be the Fifth Doctor.  As in Doctor Who.  He's never named, because that would be copyright infringement, but the frickin' Doctor is in this.  Creatures on Rirath B are also surprisingly non-Humanoid.  It's a knock on a lot of science fiction that we can only come up with alien races that have two arms, two legs, a torso and a head.  None of them are truly alien, y'know?  These are alien.

The main characters of the first two books are almost afterthoughts.  They meet up with Dairine (the little kid sister who gets an Apple laptop delivered to her that allows her to become a wizard; did I mention this is a series about kid wizards?) at the end on the planet with the sentient silicon plate.  I can't recommend this book enough, and I cannot recommend that you read this book before you read the first two in the series, as I have yet to do so and this was mind-blowing.