authorize vendor id error?

I am trying to download an e-book from my local library to my pc and the either open it or xfer it to my e=reader(Nook) the download from lib id ok but when i

try to open it i get not authorized message ...this computer is not authorized to open this book beacuse this book is authorized to a vendor id.. when i attempt to authorize it

I get another error message what do I do now?