Yoru wa Betsu no Kao

Amelia is a typical bookworm librarian. But she has a secret. At night, she works as a provocatively dressed waitress named Amber. One night as she's working as usual, she spots Tyler, the town's most sexy man, and the guy Amelia has been admiring for sometime. As soon as he sees her, he approaches her, not realizing that she is, in fact, Amelia...!

(Source: Mangafox)