An Exorcist: More Stories, by Gabriele Amorth. This book, which I read and studied recently, has immediately zoomed to the top of my list. It is an amazing treatment and explanation of exorcism and its surrounding, peripheral issues. In my opinion it is far better than his first book (below), which is excellent.
An Exorcist Tells His Story, by Gabriele Amorth
Interview with an Exorcist, by Fr. Jose Antonio Fortea
The Roman Ritual: Volume II, in Latin and English