before(:each) do
  let(:article) { Article.new }
  let(:author) { Author.new }
end

You will get:

Failure/Error: let(:article) { Article.new }
        `let` is not available from within an example (e.g. an `it` block) or from constructs that run in the scope of an example (e.g. `before`, `let`, etc). It is only available on an example group (e.g. a `describe` or `context` block).

It should be either `let` or `before`.
In any case `before (:all)` can be pretty dangerous. Someday you may shoot yourself in the foot.
The main idea is that all tests should not depend on the results of others.
Using `before (:all)` objects `@autor` and `@article` are not recreated for each test.
That being said, if attributes of these objects are changed in some tests, that can affect results of other tests.