ibooks mac keeps prompting for apple id for authorisation

iBooks on MacBook Pro using OS X Yosemite (10.10.5). Whenever I try to open a downloaded book it prompts me with:

upon entering my details and when I try to open a book again it prompts me with the same window. It also won't let me download a book from the ibooks store or download a book bought on an iOS device, instead it prompts me with the "and unknown error occurred" window. Please help I have searched this community and no one has the same problem.