When you are resident in the US, you pick up a US IP address. When you are resident in Saudi Arabia, you pick up a Saudi Arabian IP address.
Thus, when visiting the US, buy all your eBooks and Apps and then use them offline in Saudi Arabia.
There is a site where you can change your IP address here:
However, it may not work for what you want, since anything you download, may go to the provider's server. Try a free app and / or free eBook first to make sure it works. Plus, some nations make the using of a different IP address from your real location a CRIME. Thus, you must check Saudi laws before even trying that.