Planesrunner

I became aware, about halfway in, that this was a "YA" novel - and I hesitated a moment.

Then I thought bugger it, and dived back on in. I also bought the sequels from Amazon.

It is THAT good: while the hero is a teenaged kid - of Punjabi origin, brought up in London - the themes are adult, the science is impeccable, and it is a rollicking good read.

Oh, and it's serious steampunk: really, really cool airships, with very plausible technology; an alternate Earth with a coal-based rather than oil-based materials and power economy.

And did I mention how the physics is really good too? With a hero named after Hugh Everett, who originated the Many Worlds school of quantum physics, things were always going to get pretty eye-watering in the way of understanding how transitioning between universes was going to work, but McDonald does it effortlessly.

His characterisations are excellent too, if a little exaggerated in some respects (hey it's YA SF!), and it all makes for superb SF. YA or OA.