Before the Big Bang for their iPhones, iPads or iPod Touches (you can see it here)... and I am very jealous. Because I can't.
The problem is the intensely messy tangle that is the way book rights work. Before the Big Bang is published by the superb St Martin's Press in New York. They have world rights, so can sell it wherever they like. But US publishers don't usually sell into UK bookshops, because the traditional approach was to sell subrights to a UK publisher who brings out another version. When this doesn't happen, as is the case with B4tBB, Amazon sensibly sells the US version in the UK. So Amazon.co.uk is doing a roaring trade in the book, other online stores like Waterstones online do list it but typically have a 2-3 week wait while they get a copy from the States, and bricks & mortar shops don't have it at all.
When it comes to ebooks, it's generally possible to buy an ebook anywhere if the rights support this - but Apple is clearly taking more of a Music/DVD approach, so isn't allowing a US ebook to be sold in the UK. Which, frankly, is a pain.
I'm particularly disappointed because it means I can't buy a copy. I'd love to have B4tBB on my iPhone, but iTunes won't let me. (Note, 'buy a copy'. Although authors get free copies of all editions of their physical books, it seems we aren't allowed ebook freebies. But I would have bought one if I could.) Still it doesn't take away from the fun of it. My very own iPhone app. I feel like a proud parent.
Image from iTunes website