We moved into a house with an Aga 18 months ago, and without this book I would have been clueless. Agas are wonderful beasts and, as the cliché goes, I now couldn't live without it. But that is entirely thanks to this book. It's small and to the point, but I have found that I have not needed to refer to the stacks of Mary Berry books that were given to me - this little number has enabled me to adapt all my old recipes to Aga cooking without having to learn Aga-specific ones. We successfully planned our enormous Christmas meal using this book and the turkey was cooked to perfection! It also tells you all the useful non-cooking things - like how to clean it, iron with it, etc, etc.
I would say this book is essential for those who've never owned an Aga before, and jolly useful for those who have.