Co-author is a BARE ROLE NP (bare role noun phrase). Bare role NPs are noun phrases that occur without articles or other determiners. They nearly always describe some specific function, role, job or title. Here are some more examples:
- Bob was best man at my wedding.
- They elected Bertha treasurer.
- Who will be master of ceremonies?
- He was king of all he surveyed.
Bare role NPs are very different from other types of noun that occur without articles. For example, they are very different from noun phrases with normal proper nouns (i.e. names), uncountable nouns or plural nouns.
The most important difference between bare role NPs and these other types of noun phrases, is that bare role NPs can only do a small range of grammatical jobs in the sentence. They only have a few syntactic functions. For example, bare role NPs can be Predicative Complements, but bare role NPs cannot be Subjects or Objects in a sentence. Proper nouns, uncountable nouns and plural nouns can do all of these jobs.
In the four examples above, best man, treasurer, master of ceremonies and king are all Predicative Complements. This means that they aren't a Subject or Object in the sentence. Rather they are a Complement of the verb which gives us more information about the Subject or Object. These words describe the Subjects or Objects of the verb.
If we want to show that a word or phrase is a bare role NP, there are some tests we can do. Firstly, does the word describe some sort of job or role? If it does it may well be a bare role NP. Secondly can we use this word or phrase as the Subject or Object in a sentence? If this noun (or noun phrase) can function as Subject or Object in the sentence, it isn't a bare role NP. If it can't, it may well be. Let's try this with co-author and compare it with some other types of noun phrase:
- Bob is great. (Proper noun)
- Water is great. (uncountable noun)
- Tigers are great. (plural noun)
- *Manager is great. (wrong - bare role NP)
- *Co-author is great. (wrong - bare role NP)
- We love Bob.
- We love water.
- We love tigers.
- *We love manager. (wrong - bare role NP)
- *We love co-author. (wrong - bare role NP)
He is co-author of three SQL books.
The evidence further above seems to show that co-author is a bare role NP. First of all, in the Original Poster's example, co-author describes a particular role, or job. Secondly it is a Predicative Complement in the Original Poster's example. The term co-author describes he (Peter Gulutzan), it does not introduce a new person into the conversation. Lastly we have shown that the phrase co-author cannot function as a Subject or Object in a sentence. So the reason that co-author does not have an article here, is that it is a bare role NP. These noun phrases don't use articles.