Often I come across the term "female author" rather than "authoress". Which is the correct usage? "Female author" sounds wrong to me as other words that end in "-or" take a sex.
The term exists but it's considered "old-fashioned, sexist and patronising", as it says in my dictionary (NOAD):
-ess: suffix. Forming nouns denoting female gender.
- ORIGIN from French -esse, via late Latin from Greek -issa.
- USAGE The suffix -ess has been used since the Middle Ages to form nouns denoting female persons, using a neutral or a male form as the base. [...]
In the late 20th century, as the role of women in society changed, some of these feminine forms became problematic and are now regarded as old-fashioned, sexist, and patronizing (e.g., poetess, authoress, editress). The ‘male’ form is increasingly being used as the ‘neutral’ form, where the gender of the person concerned is simply unspecified.
If you want a further reference also the OALD says the same thing.
I'd say both are acceptable, though female author is probably more common. Authoress is now a bit old fashioned, I believe; it evokes a hint of romance and high literature to me, often in an ironic sense. In most cases, there is no need to express the sex of an author outside the use of pronouns (U.G. Liness uses this technique in her latest novel...): most people would simply use author for either sex.
I think gender is a bit more modern than sex, but, again, both are fine and quite common. In grammar, however, it is now nearly always gender.
This is surely the same as the Female Actor or Actress question.
If you must specify the sex of the writer, then either is OK as far as I can see (although "authoress" probably sounds quite old-fashioned to most people in a way in which "actress" doesn't). On the other hand if whether or not the person is female is not a significant issue, then why not just stick with "author"?