I'm currently reading Be My Banshee by Joyce and Jim Lavene. This book is the first in their new Purple Door Detective Agency Mystery series.
A purple door often signifies that a witch resides inside, and thus is the case with the Purple Door Detective Agency. Sunshine Merryweather is a witch who is seeking to add another magical partner to her agency since her lover and business partner was savagely murdered. Aine is a member of the beane sidhe, also known as banshees, who has come to the agency for help finding the last member of the O'Neill family. With the insistence of Mr. Bad (not his real name), another agency partner, Aine and Sunshine agree to work together to try to solve both of their problems. As fate has it, the detective investigating the murder in question is none other than Aine's O'Neill. Will O'Neill be able to see Aine for who she really is? Will Sunshine discover who killed her lover and wreak vengeance upon him? Will we find out who or what Mr. Bad really is?