"Is" or "was" written by?

We usually speak of the events of a work of fiction in present tense, even though they may clearly have happened in the past: "Macbeth hallucinates a dagger floating before him." This is because the events of the work in a sense unfold as you're reading them, so they are always in the present.

But does this extend to facts about the work itself? Specifically, does it extend to discussion of the work's authorship?

Which of these sentences is more correct?

• A Brief History of Time is written by Stephen Hawking.
• A Brief History of Time was written by Stephen Hawking.

Does the age of the work in question matter?

• Macbeth is written by William Shakespeare.
• Macbeth was written by William Shakespeare.

Does it matter if the work is an ongoing series?

• Welcome to Night Vale is written by Joseph Fink and Jeffrey Cranor.
• Welcome to Night Vale was written by Joseph Fink and Jeffrey Cranor.

Are there other subtleties I haven't thought of?

Although both usages are fairly common and unlikely to draw much attention, I would favor was written by in almost all cases. As you say, present tense is often used when discussing the plot of a work, because it helps place the reader "in the moment" as the plot is unfolding. When discussing the creation of the work, however, we speak from the point of view of the real world, rather than of the plot, and in the real world the work was created in the past and is best referred to using the past tense.

Is written by is more appropriate when applied to an ongoing series of works, although I would probably feel most comfortable using it to discuss a true periodical work, as opposed to (e.g.) a series of discrete novels: "Mary Worth is written by Karen Moy and illustrated by Joe Giella."

When referring to a specific work, you most likely want was written by.

Is written by would appear more in general claims:

• Requirements are written by architects.
• Code is written by developers.
• Documentation is written by technical writers.

What we are talking about here is not a choice between the present and the past. Both expressions contain the past participle 'written' and are in the passive voice.

So 'Macbeth is written by Shakespeare' is by no means a present tense. A present continuous could be created by saying 'Macbeth is being written by Shakespeare' which is wholly wrong as it was written over 400 years ago.

So whether we say, 'it is written' or 'it was written' both recognise that the writing took place in the past. The past context is supplied by 'written'.

A more experienced grammarian than me, will be able to describe the grammatical construction better.

In my view it matters not which one you use.