When you download a file from the iBookstore it will be in the ePub digital format, the leading format for reading books digitally at the moment. This means that you can interact with any type of book on your Apple device so long as it is in the ePub digital format, meaning you can read pretty much anything on the go.
To do this you are able to sync files from your computer to iBooks via iTunes, or you can directly purchase off of the iBookstore. You can also use PDF files which you can also transfer in a similar way, through using iTunes on your computer. These have to be manually synced to use on your devices.
As well as transferring and syncing files via iTunes on your devices you are also able to use the iBookstore on your computer, so that you can purchase files to then appear on your bookshelf, which is then accessible wherever you are. You will need to have an internet connection to perform sync on your devices, but once this is done you can access all content offline, wherever you are and whatever Apple Device that you use. Automatic download also allows you to instantaneously have files on multiple devices.