I'm going to start with a quote from the movie Gladiator:
My name is Maximus Decimus Meridius, commander of the Armies of the North, General of the Felix Legions, loyal servant to the true emperor, Marcus Aurelius. Father to a murdered son, husband to a murdered wife.
As you can see, he is a commander, a general, a father and a husband, but none of these requires an article.
When a description of a job role is unique, or when you have already named one specific person who holds that role, you can optionally omit the article.
This requires the indefinite article because there are many authors.
- He is the author of Farenheit 451.
Because there is only one author of that specific title, the definite article can be used, or it can be omitted:
- He is author of Fareheith 451.
You can also omit the article if the person is specifically named, for example:
- Ray Bradbury, author and screenwriter, died in 2012.