One of the strength of Ruby on Rails, is that it gives you "instant gratification". This is not intended to give you a false sense of ease. With Ruby on Rails real programming is required for anything more than basilar. But, in a first approximation, you can cut off all the details and see something working, having your hand dirty. Assuming that (at the beginning) you use the packages from your Linux distribution, you don't have to struggle for hours/days with installation/configuration.
As Curtis said, you will need a good understanding of many Ruby topics, but you can learn as you go. Ruby and Ruby on Rails don't ask you to conform a never-ending list of incomprehensible rules, you will find them both quite natural. In J2EE you find a LOT of contradictions, even absurdities, a name of a class or package, often mean its contrary or something else. Not so in Ruby, it is built for human beings.
That said, let me suggest some books.
One is the seldom cited "Head First Ruby on Rails". Not HF books are great, but this one is very good in my opinion: you will not be buried by repetition, every chapter is focused and follow a progression, with some useful iterations.
HF is the thing closest to a live course you could find, and it would give a good, well done, and funny, boot kick.
A slightly more advanced book (good to continue the track) is O'Reilly Learning Rails, I found it very useful, with many "real-life-like" examples. It also helps you a bit in understanding Ruby while you need, but not too much.
For that (and you will need that) I find that the best book at all, one of the best technical ones I ever read, is "The Well Grounded Rubyist": you will be surprised both by the depth of Ruby, and by the ability of the author to help you in getting it. If you ever approached Java (web or not) you will find yourself smiling more than a couple of times reading it.
Then you could end to Agile, but you will be better prepared to get the maximum from it.
And don't forget to follow RailsCasts, they are very good learning material, and they will give a lot of inspiration.