This book can you help you to understand what PhoneGap is and what can do to help you achieve your projects.
It is a reference book, to correctly understand APIs available in this framework and to show how you can simply create an app using web development paradigm and then deploy to different mobile platform such as iOs, Android, Windows Phone and so on.
But be careful, because this is not a magic wand. Platforms are different and even if developers and the communities are contributing to develop this framework, some differences persists between this mobile worlds, and so there are a lot of compromises you could accept to develop using PhoneGap.
It is anyway a great idea, and maybe in the near future, a more mature version will allow full capabilities on all devices.
This book was written when PhoneGap 1.0 was release. At this moment the last version is 1.4.1, so changes are in progrress and maybe something in this guide could be no more actual. But it covers all most significant APIs and explain with examples every single capability, so I can affirm that is a good deal to have this book to read it before, and to put it on your desk to use as a reference guide after.
If you are interested, the book is available here.