The first non-fiction book I read for fun is probably still my favorite. I used to hate nonfiction books, but randomly picked up Dead Mountain: The Untold True Story of the Dyatlov Pass Incident one day. A group of experienced mountain climbers died on a Russian mountain in very mysterious circumstances, leading to all kinds of wild theories from the KGB to the supernatural.
The author essentially becomes a detective, and the book alternates between his experience piecing together the mystery and the journal entries of the group that died. It’s fascinating and was impossible to put down.
It sparked my love of non-fiction and I have since read dozens of others. I left the book a glowing review on goodreads and the author actually liked my review, I fangirled for a bit ngl.