'solved' but not really ... just a track I hope ... 

If I add the ObjC files for class Book as listed on the tutorial-site
to my project, and have the ruby-class Book inherit from that, it
works ...

Book.rb:

ns_import( 'Book' )

class Book < OSX::Book
    #attr_accessor :title
    #attr_accessor :author
end

I'm not knowledgeable of RubyCocoa intrinsics, but this quirck tells
me somewhere along the way the KVC-resolving gets confused, as
apparently the accessor-methods for the keys do not translate or map
to the ObjC wrapper and core object for Book ... it might just be a
naming-question ... ?

any comments welcome,
Harry