The Fourteenth Letter - Claire Evans 

 

To paraphrase the Red Queen: "Off with his head". This is for Hermiones teeth! (As you can tell, at the moment I´m not a big Snape fan).

 

This book fits all the required tasks. It published after 1990, the last name of the author starts with an E and the female main character of this novel is a morally ambiguous character. As a matter of fact, I liked her a lot.