When using the iBook store app you are able to read content that you have purchased on several devices at once, using sync technology and automatic downloads. This means that whenever you purchase a book on any device, you are effectively buying it for your account rather than one device in particular, so that you can then access it in multiple places at once.
As it is automatic this means you do not have to do anything – whenever you use the iBooks app on any supported Apple device and you log into your account, all of your downloads will be there waiting, so that you can pick up from where you left off and keep on reading.
A note will be made of where you last got to so that you can then easily carry on reading from one device to another, without having to worry about finding your place again. You can also Archive books so that they take up less space and are not immediately accessible on a certain device if you do not want to have a certain file on a particular device, so that you can keep track of what books and files you have on which device.