"Is" or "was" written by?

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.

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."

Well, no. We often use either the past or the present, while the events may well be in a hypothetical future ("The Daleks invaded earth in 2150").

When we do choose to use the present here, it's a case of the historical present, which as well as being used in summarising fiction, is also used of actual events, and within fiction itself.

Two places where its use for actual events is particularly common are in summarised timelines of events, and "breaking" news headlines.

It can be used much more extensively, and doing so is recently popular; what was once the rhetorical technique of translatio temporum where one uses the historical present to foreground particularly interesting parts of a narrative, is now increasingly the norm. As a matter of style, some love it and some hate it, but either avoiding or embracing can be done in technically correct English.

Now, when you come to write or speak about who wrote a novel or play, you are of course relating real rather than fictional events* and so the same applies here; it's particularly common to use the present if you are doing so as part of a summarised timeline, or it's "breaking news", and otherwise while increasingly common, it's a style some dislike. Otherwise the past tense would be more usual.

*Unless perhaps you are writing about Stephen Dedalus writing about Ibsen or Pierre Menard producing his version of Don Quixote etc.