I read this because I wanted a decent account of the true story on which The Exorcist was based, and that's what I got. The author is very plain about his sources, and how he has used them to write his book. The key source is a diary-like manuscript, recording the exorcism process (which lasted quite some time).
The full manuscript is reproduced in the book, and when I came to read it (having already read the main story), it was very obvious that the main narrative of the book is little more than an embellished, fleshed-out version of the manuscript. I suppose this was to be expected, and undoubtedly the author's embellishments add value, but if you are just mildly curious (rather than seriously interested) it is not neccessary to read both, and just reading the manuscript itself will satisfy your curiosity much faster than reading the bulk of the book, whilst also leaving you freer to draw your own conclusions.
While not a fantastic piece of writing, this book is unique, and as such has no competitors against which to be judged: it does what it says on the tin, and you are either interested in the contents, or you aren't. If you ARE interested, then I recommend this book without reservation.