“a yearbook”

“Y” is used as a consonant sometimes, like in “Yankee” or “Yucatan”, but it’s also used as a vowel like in “fly” or “shrimply” or “Yttrium”.

“a” and “an” are chosen based on the pronounciation, not the letters, so it would be “A yearbook” or “A Yankee” but “An Yttrium bar”.

The “n” is added to avoid the difficulty of pronouncing two vowel sounds back-to-back