A three-novel series is a trilogy. A four-novel series is a tetralogy.
What do you call a series of two novels?

It doesn't appear in any dictionary that I know of, and language
purists are not likely to accept it, but "duology" is sometimes used:

://www.google.com/search?biw=772&hl=en&q=duology

A tetrology is also often called a quartet

I think trilogy decomposes as tri- (three) + -logy (discourse,
according to the OED).

Original meaning :
"A  series  of  three  tragedies performed in Athens at festival of Dionysus"


Therefore, one could write series of two works as "dialogy" (rather
than -- at least I think -- "duology"). I'm not sure anybody actually
does this however.

I agree that "dialogy" is a more pleasing construction than "duology,"
but "duology" has the advantage of being in actual use.

Duology is improperly formed, having one foot in Latin and the other
in Greek. But that doesn't completely rule it out as an English word.
After all, nobody minds television, a hardy hybrid which exists
because telescope was already taken, and transvision--which would have
been acceptable to purists--doesn't seem to carry the right sense.

Anyway, I'm happy with duology. Thanks.