resources :series

rake routes | grep series

series_index GET /series(.:format) {:action=>"index", :controller=>"series"}
series_index POST /series(.:format) {:action=>"create", :controller=>"series"}
new_series GET /series/new(.:format) {:action=>"new", :controller=>"series"} edit_series GET /series/:id/edit(.:format) {:action=>"edit", :controller=>"series"}

  series GET    /series/:id(.:format)      {:action=>"show", :controller=>"series"}
  series PUT    /series/:id(.:format)      {:action=>"update", :controller=>"series"}
  series DELETE /series/:id(.:format)      {:action=>"destroy", :controller=>"series"}

So, you should use new_series_path and not series_new_path.