Brief redux:

You should know enough to be comfortable with some basic to
pre-intermediate stuff (in practice this means you don't need to refer
to the manual every 5 minutes if you write a bit more complicated stuff)
This should be acquired from Ruby for Rails (IMHO - there are lot of
other ways to begin, but for me this was the best)). If you need some
more, you should grab The Ruby Way and/or the Pickaxe, write a lot to
ruby-talk etc.

Once you are ready to roll your own project (which is IMHO the second
step after you have some knowledge under your belt), nothing beats the
Ruby Cookbook. I have done this myself recently. I had no idea about
documenting Ruby code, packaging the whole program into a gem, writing
unit tests (in Ruby) and automatizing these tasks (and a lot of other
things - not to mention I had no clue about Ruby-ish constructs and
idioms). However, with the Ruby Cookbook by my side, learning and
putting things into practice from writing the first line until packaging
the whole thing into a gem was a piece of cake. I have picked up a lot
of idiomatic thing as well - and I think the cookbook is the best stuff
for this.

Of course If you want to just code some smaller stuff and refer to the
cookbook sometimes, it is cool for that purpose too. It has all the
typical problems that a Ruby hacker encounters.

Conclusion: The Cookbook really shines if you are actually doing
something and would like to do it the Ruby way. Reading it from cover to
cover from the beginning might also work - never tried :)

Cheers,
Peter

p.s.: I will also send the full review once I am ready with it.