a versus an

Something that's been bothering me (that as an English teacher I feel I ought to know) is the use of 'an' with a word starting with 'h'. In a story I've just finished, I was talking about the hysterical note in someones voice. I wrote 'an hysterical note' but the spell checker thought it should be a hysterical note. I am reading a book at the moment: Patrick Gale 'A Perfectly Good Man'. In it he uses 'an historic' so I'm getting rather confused as to the rules. Any ideas?