Question about the RoR anti patterns book

Would you say that its a good book for RoR beginner?

I'm not the author, so take what I say with a grain of salt.

I don't think that beginners will be able to benefit from the book as much as the book deserves, so to speak.

Basically, if you have written a few rails applications, you will recognize several things:
1. You see the anti-pattern, and recognize something you did (along with the reasons you chose to do it)
2. You understand the discussion of why it's not really a good idea afterall
3. You get the benefit of a nice a-ha moment when you see how to solve the problem in a different way

If you don't have any experience with rails, there's a chance that much of the discussion will just be words.

Said another way: if you've made the mistakes, the pain you felt making the mistakes will make it that much more efficient to learn from the book.

Were these antipatterns observed in the "wild"?

You bet!

They are naturally occurring anti-patterns observed in their natural habitat. Fascinating creatures...

If you mean practically, then yes, these anti-patterns have been picked up from live/real world rails implementations that have gone wrong.
As RoR is maturing as a platform, we need to be aware of the common mistakes that occur in it (by java, php or .net developers) and have a roadmap for refactoring our existing codebase.

Chad and I have had a lot of exposure to "interesting" rails applications as consultants (Thoughtbot, and independently). These AntiPatterns are definitely issues we've actually seen - no armchair philosophy coding, here