If you can get to London, it's worth taking the IT1N form in person. The US tax office is at the American Embassy.
That's what I did, and I didn't have to leave my passport. They took a copy of it and my publishing contract, handed it back, and I was in and out in ten minutes.
Shara
That is what I intend to do... looked at all the other options and they all involved either sending my passport away to the US (hmmm, not an option I am happy with, thank you, especially given the several months turn around), finding a notary in the UK who the US will accept (expensive from what I have seen and still involves travel to another city) or going to the embassy in person. Since going to the embassy will involve, basically, a train ticket (as opposed to a train ticket plus several hundred pounds to pay the notary) it seems like the best option.
Todspace: All you need to do to apply for a ITIN is have a letter from the publisher (or a contract or whatever - something official that proves you have a published work). You can do that as soon as you get an agreement (not sure how Amazon handle this, presumably you can get them to send you something?)
One thing to be aware of... I've been told that its not just the 30% tax that gets charged if you don't have one, there is also a fine for the publisher if they knowingly let you continue without one and they get audited and the IRS find they don't have a SSN or ITIN or equivilent listed for every person contracted to them. Apparently, this has been in place for a few decades but, pre financial crisis, they rarely bothered to check publishers (especially small press) so a lot stopped doing it. Now they are tightening up a lot, presumably to regain some cash in those 30%s of royalties and publisher fines.