I want to buy only one book, which one would you recommend?
It has to be RoR 3.0 (its OK if the book is not published yet, RoR 3.0 itself is not released yet), I can do a pre-order.
Its OK if the book doesn't deal with Ruby (in fact its better), there are quite a few good free online Ruby books online.
Agile Web Development with Rails from The Pragmatic Press is pretty much the canonical Ruby on Rails book. Version 4 of the book, covering Rails 3, is available for pre-order as well as a beta book, which lets you download a PDF of the current version, and also get a hardcopy once it's finished.
http://pragprog.com/titles/rails4/agile-web-development-with-rails
This will probably end up being the defacto Rails 3 book. And the nice thing is you can get the beta ebook now (and subsequent updates), and have the "real" one sent as soon as it's out.
If you already know Rails, start with http://www.railsupgradehandbook.com/?r=rubyinside to get the basics of Rails 3 and how to upgrade your apps and your knowledge.
If you don't yet know Rails, the book mentioned above makes the most sense, but you can also get a good grounding online at http://guides.rails.info/getting_started.html