Some months ago, a person from Packt Publishing posted a message in this list looking for potential Authors/Reviewers for new Ruby on Rails books. Now I'm proud to announce my book  "Aptana RadRails: An IDE for Rails Development".
If you are using  Aptana RadRails, or if you are looking for an IDE and you still haven't decided which one to use, you might be interested.
It's my first book, so of course there is a lot of room for improvement; but it can come in handy, specially if you don't have any previous experience with the Eclipse interface, which can be a bit intimidating at first.
If you want some background information or want to get in touch, you can visit my profile on  workingwithrails
p.s. I don't pretend to start a flame war about IDEs here. I've been happily developing RoR projects for 2 years with RadRails, but whatever floats your boat is OK with me. In the end is just a matter of choosing and it's a good thing there are so many options available.