If you're looking for more general but advanced books there are loads of fantastic ones - some of my favourites are Eloquent Ruby, Design Patterns in Ruby, Metaprogramming Ruby and there's Crafting Rails Applications for Rails (though I haven't started this myself yet - but I hear it's very good).
If you're looking for books on the internals of Rails, this one could be right up your street: http://patshaughnessy.net/ruby-under-a-microscope
If you're looking for more general but advanced books there are loads of fantastic ones - some of my favourites are Eloquent Ruby, Design Patterns in Ruby, Metaprogramming Ruby and there's Crafting Rails Applications for Rails (though I haven't started this myself yet - but I hear it's very good).