Help! 10 GB Memory Gone POOF! Ibook keeps downloading over and over and won't let me delete it.

Someone started a download of a textbook on my Mac and I clicked to stop the download. I tried to delete it but it wouldn't let me delete it. It was at 1.68 GB of 1.72 GB So I finished downloading it, all 1.72GB of it and I clicked delete. It wouldn't let me delete it. It started downloading again from the beginning! Over and over again each time I tried to delete it. Finally, I opened it to see if it had actually downloaded and it had. Then I clicked to delete it again and It asked me if I wanted to delete this book, I clicked yes. Then it started downloading the book again. So far I estimate it has downloaded about 10GB of my Mac's space and the book was originally on the computer on 5-2015, so it's been here a while, I just don't use iBooks so I didn't notice it. I can't get rid of it now. It just keeps eating up more and more space. I tried looking for a file folder to delete from but I can't find any. Please help. This is my first Mac and I've hardly put anything on it except one movie and a few pictures and suddenly POOF 50 GB of space is GONE! What is happening?!?!