When using iBooks you will come across both DRM files and DRM-free files, which will limit what you are able to do with a certain book and whether or not you are able to freely copy the book around. This is put in place to prevent people from sharing files with others who have not paid to view the content itself.
Not all content will have DRM however, and iTunes can tell you whether or not the books you have is DRM protected or not, simply by going to File and Get Info. This will tell you whether or not this is DRM protected – if the ‘Kind’ option is marked simply as Book then this will represent the fact it is not downloaded from the iBookstore and is rather a book which you have obtained from another site on the internet.
A Purchased Book on the other hand is a DRM-free book which has been downloaded directly from the iBookstore, whereas a Protected Book will represent books which have been purchased from the iBookstore but are copy protected, in order to prevent copyright infringement and the like of the book. It is up to you to see what the file type is and to act accordingly when using it.
To make things easier for yourself you might want to just purchase on the iBook store – you can verify up to five devices from one account which makes it much easier for you to read the content you’ve paid for. Amazon’s Kindle application, however, is also very painless and nice to use.