Grace Abounding to the Chief of Sinners by John Bunyan was published in 1666. Bunyan wrote this book while he was serving a twelve-year sentence in jail for preaching without permission. This book is such a vivid account of the struggle Bunyan went through coming to faith it will make your heart ache. The only other faithful brother I’ve heard likewise is Martin Luther, who seemed to suffer from a similar burden of sin.
This book, like many other’s I’ve recommended to you, is in the public domain. This means you can download a free pdf or Kindle version, listen to an audible book, or purchase your own copy. I would strongly recommend this book to go along with the reading of The Pilgrims Progress.