I found an interview of Ahmad Deedat in youtube where he said that this book changed his life. I found a pdf version of it online but I try to never really look for information about islam online just because I'm afraid of getting false information.
The book is Izhar-ul-Haq by Mohd. Rahmatullah Kairanvi. It was the first link I got when I googled the book so I'm sure someone has seen this pdf before.