Sure, but there's also 'an historical novel' - less likely, unless you are a character in said novel. I would say that the real rule is not about the letters (

Sure, but there's also 'an historical novel' - less likely, unless you are a character in said novel. I would say that the real rule is not about the letters (vowel or consonant), but the sounds. So, you receive 'an MBA' because it's pronounced 'an em-bee-eigh'.