Isaac_Newton got unlucky with the day he looked up the price on Amazon UK. They have been playing silly beggars with the price of the HC from 6-18 July. Now it's back down to £15.90. Their lowest price promise applies and means you pay the lowest price they list between when you order and they deliver. I certainly haven't paid publisher's list price for any of the Safehold books I've bought from Amazon UK.
Factoring in delivery costs (12-17 Business days!) the cheapest I could buy it from Amazon US today is $22.50 (£14.87). If I get it from Amazon UK it will cost £15.90 ($24.05) and I'll get it the day it's released. Guess which I'd choose
Actually, I've been really mean and bought the Kindle edition for £10.44 ($15.79) and saved 46c on the US price! (and that's including the VAT)