It is an interesting coincidence that I was just telling some friends about this book today and later found it is the free book of the month. Needless to say my friends will be getting a copy!Bunyan was a imprisoned for 12 years for preaching. His captors were willing to release him if he would but agree to cease his preaching. Though his family too suffered intensely due to his captivity Bunyan would not agree to these conditions for his release. Bunyan wrote an abundance of books during this imprisonment. One of which (if I remember correctly) was this wonderful Christian allegory The Pilgrim's Progress. I have heard it said that this book is the most widely spread book in the world apart from the Bible.
For more details on Bunyan's life I would highly recommend Piper's biographical message on Bunyan which may also be downloaded for free here.