Not a sourcebook, but helpful to mine for ideas--
Butcher's Summer Knight
Feist's Faerie Tale
Goldstein's Strange Devices of the Sun and Moon
You could get by with just the core monster books-- it's all a matter of presentation...
Remember, these are the beings that steal children, dye their hats in your blood (Redcaps), tempt you into drowning (kelpie), kill you with a kiss (the White Lady), and pay a tithe of human souls to the devil every seven years...
Many of the D&D monsters have a basis in faerie legends as well-- the greenhag (Jenny Greenteeth) and the troll (which in Germanic folklore is a dwarf-like sorcerer, transmutation specialist)...
The "Tithe to Hell" idea might work for you, especially since in many stories it's the Seelie Court that has to pay, to ensure the freedom of the Faerie Lands from the Devil. If your players are expecting a "Good Faerie Queen", or if all they know is "Titania" from Shakespeare or "Tinkerbell" from Disney, it should be a big surprise...