Merriam-Webster's Concise Dictionary of English Usage: a, an

(I'm going to read this front-to-back and I learn best by writing, reading and restating...)

Merriam-Webster's Concise Dictionary of English Usage: The Essentials of Clear Expression

The basic rules are these: use a before a consonant sound; use an before a vowel sound. Before a letter or an acronym or before numerals, choose a or an according to the way the letter or numeral is pronounced: an FDA directive, a U.N. resolution, a $5.00 bill.