As of late, it seems as though there's been an awful lot of established adult science fiction / fantasy authors publishing books specifically for YA audiences. Some have been very successful (Paolo Bacigalupi's Ship Breaker won the 2011 Printz award), some have been of outstanding quality (China Mieville latest Railsea along with Catherynne M. Valente's The Girl Who Circumnavigated Fairyland In A Ship Of Her Own Making), and a few have been, well, insultingly patronizing (David Weber's A Beautiful Friendship, whose problems require far more than parentheses). So when I saw that Ian McDonald (River of Gods, Brasyl, The Dervish House) was publishing Planesrunner as YA, I knew I had to figure out just where it fell in this spectrum.
Planesrunner opens with Everett Singh watching his father, quantum physics researcher Tejendra, kidnapped from the streets of London. Shortly afterwards, he receives an email from his father containing the Infundibulum, a map detailing how to access an impossibly large number of alternate universes (10 ^ 80th power, to be precise). Slowly Everett finds himself drawn into the secretive nature of his father's work and the interplanar diplomacy between our Earth and nine other alternate Earths, as well as the designs of Charlotte Villiers, the plenipotentiary from Earth 3, who seems determined to get the Infundibulum at all costs.
Predictably, the book is a little heavy on quantum physics, particularly the many-worlds interpretation. It's not super-hard science fiction and McDonald doesn't go deep into actual quantum physics, so you won't have to spend half the book wondering what a muon is, but he does touch on the surface-level broad theories that most are familiar with. It's an excellent, more "sciencey" companion for readers who liked the Pendragon series, and a counterpart to Pullman's excellent His Dark Materials trilogy (though the two have very different thematic aims).
Outside of the quantum mechanics angle, it's a very straightforward adventure novel, especially once Everett moves from our Earth to Earth 3—a sort of anti-steampunk world, which skipped steam engines entirely and went straight to generators and electricity—and meets Sen, the young pilot of the airship Everness. This, of course, leads to some minor swashbuckling later in the book—you can't have airships these days and not expect some buckled swashes—but despite being a fairly fast-paced book it's not action-driven. Planesrunner builds its tension and momentum through dialogue and the nature of the plane-jumping technology; it won't satisfy those looking for adrenaline rushes, but slow-boiling suspense and tension is much more effective at creating a dynamic story, and—from reading some of his adult novels—McDonald is better at that style anyway.
There's only one thing that really bugged me about the book, which is that sometimes it felt like McDonald was trying a bit too hard to be relevant to his teen readers. There's a dropped reference to TVTropes which already feels a bit dated, and a few other odd scenes that have a distinct "what middle-aged men think teenagers do today" flavor to them—nothing that really breaks the book or makes Everett a caricature of a teenager, since otherwise he feels like an actual hyper-smart teenager, but it's definitely jarring and I can see more than a few readers laughing at them. It's also worth noting (in case it's not obvious from the names) that Everett and his father are Punjabi—I'm not entirely happy with the stereotypical "people of Indian descent are naturally good at STEM fields" vibe it has going, but those actually of Indian descent can comment on that better than I can.
Planesrunner is definitely one of the better outings for adult SF authors in YA—SF's more adventure-driven side has always been the most successful with younger audiences (occasionally to the chagrin of SF authors) and McDonald plays well upon that. It's also the first of a trilogy, continuing the time-honored tradition in SF to make everything a trilogy, so hopefully the remaining books only improve.