Books can be attributed to "Anonymous" for several reasons:
* They are officially published under that name * They are traditional stories not attributed to a specific author * They are religious texts not generally attributed to a specific author
Books whose authorship is merely uncertain should be attributed to Unknown.
Luke was a doctor. He wrote truth and reported it so that we would be able to know that what he wrote was truth. The relationship between Luke and Theophilus was so strong that they only wanted the truth and Luke wanted his friend to benefit from the truth.