Ajax On Rails
mu-sly writes "Ajax and Rails - probably two of the biggest buzzwords in web development at the moment. In this article over at ONLamp, Curt Hibbs introduces the incredibly powerful Ajax support that is part of the Ruby on Rails web application framework. It's a great read, and serves as a gentle introduction to the cool stuff you can accomplish with ease using the Ajax features of Rails."
Agreed 100%. I bought it last week. Don't be fooled by its "beta" status - it's a thoroughly well written introduction to Rails that takes you through everything you need to know - including building a REAL application (a shopping cart) from start to finish.
I also bought Programming Ruby by the same author because I want to do more with Ruby than just Rails, but this isn't necessary for those who just need Rails - all Ruby constructs and idioms are cross-referenced with an included introduction to the language.
--- Hot Shot City is particularly good.
RoR has caching : here and supports transactions : here