Is there a book about RoR for someone that has little or no understanding of Ruby?
Although RoR is a great reason to start learning Ruby, I'd be bored out of my pants if I first have to read a full book on Ruby and then a book about RoR.
Something that demonstrates the basic principles of both Ruby and RoR would be an ideal intro to this world.
Agile Web Development with Rails (Thomas, Heinemeier Hansson) is a good intro to rails, and introduces Ruby gently along the way, you don't need to know ruby to read it.
http://www.amazon.com/Agile-Web-Development-Rails-Programmers/dp/097669400X
It is wise to first learn the (basics of a) language before using more advanced frameworks in order to fully make use of the framework. Same goes for Ruby and RoR.
Personally, I had the same problem. But I found a nice Ruby book (Mr. Neighborly's Humble Little Ruby Book, explaining Ruby in a none/less boring way. I thought it was actually fun to read!
After learning the basics of Ruby you can start looking at the (somewhat) official guides. They are easy to understand and shows a lot of RoR basics (and more advanced stuff).
AWDWR is a good place to start. A couple other books worth looking at are Simply Rails 2 and Foundation Rails 2. Veger does have a good point too; without a certain level of Ruby knowledge you will struggle in some areas with Rails.
I would personally recommend checking out why's Poignant Guide to Ruby: http://mislav.uniqpath.com/poignant-guide/ first to get familiar with ruby, it makes it entertaining to learn. Then for Rails I would recommend Agile Web Development with Rails.
Simply Rails 2 By Patrick Lenz is a good Rails introduction for someone with very little Ruby experience. But you will need another Ruby and Rails book or resource to really master both.
A good free alternative is Ruby on Rails Tutorial. It's a work in progress, but even the first 6 chapters helped me out a bunch.