Wizard by Trade is the Omnibus edition, combining Summer Knight, book #4, and Death Masks, book #5, from The Dresden Files series. This hardback book was published after both paperbacks and prior to the two hardcover editions, making the Omnibus edition important to collectors.
Summer Knight: Private detective/wizard-for-hire Harry Dresden is suckered into tangling in the affairs of Faerie, where the fate of the entire world - and his soul - are at stake.
Death Masks is an altogether darker work, focusing on demons rather than faeries. Harry Dresden, Chicago's only practicing professional wizard, is hired by a mysterious priest to find the stolen shroud of Turin.