Alan Davies plays Jonathan Creek, whose day job is as behind-the-scenes man for a magician ... he's the one who designs the illusions and the equipment to perform them. An allegedly "impossible" murder or robbery is just another form of magic trick ... Jonathan can look past the smoke and mirrors to see the mechanics they distract the investigative eye from.
Caroline Quentin is Maddy Magellan ... a crime writer who connects with Jonathan while trying to collect information on a murder and then keeps coming back (when Caroline left the show after a few seasons, her replacement was Julia Sawalha ... better known as Saffy to the AbFab fans).
The mysteries themselves are great, Davies and Quentin have a fantastic acting rapport, and the I-can't-figure-out-if-I-lust-for-you-or-l
Excellent fansite here if you want to learn more about the show.
EDIT FOR BONUS NEWS: Season 2 of Black Books is also slated for fall release in Region 1 (exact date not available yet)