If you want to ultimate example of allegory, this is it.  Christian leaves his home in the City of Destruction to journey to the Celestial City.  Evangelist helps him and he has companions like Faithful and Hopeful.  The meaning of this book is not exactly hidden from the reader.   

If you aren't religious, you may think the book won't interest you but if you are interested in literature in general, this is a foundational piece of British literature.  You don't have to agree with Bunyan's religious beliefs to find the beauty in this story.  Christian and his friends are on a quest and they must persevere and battle through many odds to achieve it.  All the finer qualities of humans, as well as the worst qualities of humans, are displayed brilliantly in this book.  Bunyan understood people. As a Christian, I find the book comforting to read as Christian battles through the same things we all face every day. 

The bottom line with Pilgrim's Progress is that whether you are a Christian or not, the book is a masterpiece and has literary value for any lover of British literature.