There are several places to purchase books online, but often the shipping charges are high. Next time you're perusing book selections, hop on over to Book Depository - shipping is free.
Note: Your credit card may charge a foreign transaction fee (usually 3%) so check with your credit card company first and use a card that does not charge transaction fees (Amazon Reward Card doesn't charge foreign fees and you get 3% cash back on Amazon purchases).
Also, if you're looking for a classic go to Gutenberg Project where you can download books copied by volunteers for a free download.