On this website we quote book prices in UK pounds sterling (£ or GBP).  However you can most likely use your USD or other currency credit or debit card to purchase our books.  Your bank will pay Google Checkout, who handle our e-commerce payments, in pounds sterling, then debit you the appropriate amount in dollars.  The appropriate amount will be the exchange rate at the time PLUS their charges which will probably add a few percent.

Here is a currency converter using the current fx-rate.net mid-market rates (which will not be exactly the rate your bank uses and before bank charges):