I read this book cover-to-cover as soon as I bought it. I love things that are organized and encyclopedic, and though I'm not Catholic, I love this book. I frequently look up information in it, it contains not just information on Popes, but many Vatican buldings, departments, and artwork. This would be invaluable to anyone taking a trip to the Vatican.
I have also read Bunson's Encyclopedia Sherlockiana and found it to be equally detailed and informative.
I heartily recommend these two books to anyone who is even remotely interested in either subject. After you're through, you'll be more than remotely interested, you'll be hooked.