Rob Sanheim (one of the Ajaxians) has put together a great review of Ruby For Rails by David Black. It's a great book, and if you haven't got it yet, let Rob's review convince you. I personally found it an interesting read and it's really easy to get stuck into. It's an ideal book if you've made it through Agile Web Development for Ruby on Rails and are hankering for more as it dives into the Ruby side of things a lot more.