Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
The answer to your question depends upon what website you wish to buy an e-book from.
First, go to their website.
Then, log in to your account with that website (e.g. if an Amazon.com book is wanted, log in to your Amazon.com account.)
Then buy the e-book.
That's how - then your Samsung tablet should be able to download the already-bought book from the (for instance, Amazon.com) site.
Try it... does that work?