I am a big fan of ruined castles. I like ancient, crumbling old buildings from times long past. There are many such castles in the British Isles, but there are also many more "modern" castles, built hundreds of years after the medieval period, filled with sumptuous carpets and luxurious wall coverings and such. I can see the allure of these castles for some people, but they do not hold the same wonder for me, and when I am paying a lot of money to visit a foreign country, I want to optimize my experience as much as possible.
This book is wonderful because it lets me do just that. Many guidebooks (I use the Let's Go and Rough Guides) do not differentiate between different kinds of castles, and offer a brief description of each one. It's hard to know what you're getting into before you actually arrive. With this book, you can get a preview of each castle, so you're never surprised. I mentioned my personal love of ruins, but this book is great for any taste, or just the curious.
The information on each structure is sparse, and much attention is paid to the glorious full color photographs. I never got the impression this book was trying to be the authority on castle information however, and I think it succeeds admirably at its chosen task. It is a good supplemental guidebook if you're interested in the topic.