The Pilgrim's Progress by John Bunyan
"The Pilgrim's Progress" is a Christian allegory. It follows the path of Christian as he tries to find salvation. He is a resident of The City of Destruction. After realizing he has a "heavy burden" he sets off on a quest to find peace.
I found this a very difficult book to read. First published in 1678, it was immensely popular and remained so for two hundred years. But to understand it, you have to know a quite a bit about English religious history. I don't, but the extensive end notes were essential to my understanding. If you read this book, make sure you get a copy with notes.