You read a book. You think “my friend should read this book”.
You let us know what book you want to buy and to whom to send it to. You pay for the book. The friend who gets it doesn’t know that you sent it. He reads the book. If he likes it he might want to know who you are, so he pays for the book, and we tell him it was you who sent it. When he pays, you get credit for the full amount you paid for your next gifting pleasure. Or if you want to stay anonymous even after he pays he can send you in return a book you might like.