I will suggest the following books to learn ruby on rails
One should first learn ruby since rails is based on this. There are two interesting books I would recommend
Learning Ruby, Oreilly Publishers
The book talks about the syntactical details of Ruby in a simple and practical manner with lots of example
The other book is
Ruby For Rails,David Black,Manning Publication
The book explains Ruby as well as Rails. Lot of examples are given so that will help in easy understanding of the topic. The advantage of this book is that its a complete deal it talks about Ruby, Rails and gives an example Rails application.
Refer to the above URl. This is like the first guide that every Rails beginner must refer to. The page explains how to install rails, How to write the hello world in rails. It also gives an example to complete the overview of Rails
2. Agile Web Development with Rails,second Edition:
This is the book of Rails. Written by the creator of rails (DHH) himself is the most authenticated account of the Rails framework. Usually the books written by the language creators are little tough to understand e.g. Kernighan &Richie 's C, Bjarne Stroustrup's C++, But this book is very simple to understand. The book talks about the internals of Rails framework and also develops an application. Its a must to read book
One thing I have noticed in the books that are being written these days, the authors are more inclined towards explaining the practical aspects of the language giving examples rather than just a theoretical overview.