Correct choice of a and an depends on the initial sound, not on the initial letter, of the word that follows. A should be used before all words beginning with a consonant sound except silent h (an honor) and before words beginning with vowels that represent combined consonant and vowel sounds (university, unit). An should be used before all vowel sounds.
a boy, a European, a g, a picture, a store
an entry, an f, an hour, an orange, an unbeliever
With the words history and historian, a preceding a is more often used today than an. In history, the h is always pronounced; the h in words such as historian and historical was formerly not pronounced, but it frequently is in contemporary usage. Always say "a history book," but refer to "an (or a) historian" and "an (or a) historical novel" as you please. Both are standard usage.
Neither a nor an is needed in such expressions as "no such (a) thing," "no greater (an) honor."
Dictionary of problem words and expressions. Harry Shaw. 1975.