Dante's Inferno. It was full of footnotes explaining the context for all the references and allusions he made, which was important to have, but reading your way through a piece of literature and being stopped every few sentences for a lengthy explanation was so frustrating. I couldn't keep a good pace up and kept getting lost in the details. My interest gave out and I still haven't finished the last quarter of the book.
And there are two more volumes after that in the Divine Comedy!!!
That sounds like a problem with whatever edition you read. I had to read it a long time ago for school and the version we read was more like a short novel and I found it an interesting read.