I love non-fiction books: biographies, autobiographies (I'll talk about these more on Day 25), and history books. I've read several non-fiction books for each of my degrees, but I can't say that any of them would be my favourite non-fiction book.
My favourite non-fiction book was found on my "Christian" bookshelf, the one with all my devotional books, Bible reading guides, etc. At first I thought it would be Jesus Freaks, which has dozens and dozens of stories of martyrs, from Stephen in the Bible, to people killed within years of the book being published. However, just as I was about the grab it off the shelf, I saw this book and knew it this was my favourite non-fiction book:
Heaven has a Blue Carpet is a funny, inspiring, heart-breaking, challenging book, in which Sharon Niedzinski talks about what it's like to look after sheep, and then uses that to explore what this means in regard to the Biblical metaphor of the sheep (Christians) and the Shepherd (Jesus).
As someone who grew up in a Christian household which also had pet sheep (sort of... we ate them...), I had a good understanding of the sheep-shepherd idea, but this book really brought a lot more insight and understanding. It's inspiring and encouraging and I throughly recommend it if you're struggling with understanding how much you are loved and cherished by God.