To transfer a book from one Kindle to another there are two main methods. First any purchased items can be downloaded to a different device from the Archives section of the Amazon account. Second they can be transferred onto a PC via a usb cord and then put on the new device via the same method. For more troubleshooting tips for the Kindle see: http://www.amazon.com/gp/help/customer/display.html?nodeId=200375630.
You mean share Kindle books between two Kindle device? You have several choices: you can de-register one Kindle and register it on another Kindle, then sync the books or you can strip the DRM to transfer it on both two Kindle devices.