With Amazon Kindle you can share your books with another person through a Family Library account. This option links two Amazon accounts. To set it up go to the Home screen of your device, tap the Menu > Settings > Registration > Household > Family Library > Add a New Person > Add Adult.
Once you will share it with your family/friends, your friend or family member will then need to enter an Amazon account ID and password. Once the information is entered, the second person will be asked to enable sharing. Then, you will be prompted to enable sharing, as well. Both parties can choose to share all of their purchased books or just individual books with each other.
Once thing you need to be consider while sharing though. Amazon says that by choosing to enable sharing, both parties can use credit cards associated with the two accounts for purchases on the Amazon site. So make sure you are fine with this.