Is it "A historian" or "AN historian"? I've seen both.
This is a contested question.
Let's start with the basics. A word starting with a vowel sound is preceded by "an," no matter how it's spelled. Thus, one would write "an hour" and "an honest woman." A word starting with a strongly stressed "h" sound is preceded by "a." Thus, to most Americans, it's "a hanger" and "a helicopter."
There are two questions at issue here. The first is what one ought to do with a weakly pronounced "h." Some—this editor included—would precede it with "an"; others—and this is increasingly preferred—precede it with "a." The second question is how you yourself pronounce "historian." In general, words in which the "h" has been dropped have been shifting toward having it pronounced strongly, but only you know how you pronounce this one. If you drop the "h" entirely when saying "historian," then precede it with "an." If you pronounce it with a strongly stressed "h," then use "a." If you pronounce it with a very faint "h," then do whatever seems more natural.
It's worth remembering that the reason why "an" is used before vowels is because it's hard to go straight from "a" to another vowel sound without having them run into one another. Native speakers of English almost never use the wrong one, at least in speech. Say it out loud before you write it, and trust your instincts.