The only depiction of K-Wings in a certified publication (that I have found at least) appears in The New Essential Chronology. The bomber consists of a central wing with two V-shaped ventral wings protruding to form a K-shape. The bomber also has two cockpit canopies in the front as well as a ball turret near the nose and on top of the canopies.
Here are some computer generated models and artist depictions:


The second image is the one that most closely resembles the K-Wing depicted in The New Essential Chronology.
The K-Wing appears to take elements of both the Republic LAAT, Separatist gunship, and the Y-Wing. Is there any other mention of the K-Wing outside of the BFC in any other book?