The Case of the Missing Spot — Stephen W. Martin

Intrepid eight-year-old detective Nancy Spector and her talking dog, Jinx, search high and low for an invisible dog in this laugh-a-minute, fast-paced graphic novel perfect for fans of Dungeon Critters and The Mighty Jack.

The Invisible Man has a problem. His dog, Spot, is missing. Complicating matters, Spot is also invisible. So the Invisible Man turns to Nancy Spector for help. Nancy eagerly takes the case, and she and her grumpy best friend, a (visible) dog named Jinx, dive into the investigation.
 
As they follow Spot's trail, they find themselves: Riding dragons Searching the monster diner -- because all dogs like hot dogs Fleeing from vicious chipmunk unicorns Shopping in a store that is a gelatinous candy cube named Bob  And paying a visit to Nancy's vampire friend Eugene's hamster's funeral  Every time they think Spot is right around the corner, things get more and more confusing, but Nancy knows that to a great detective, there is no such thing as a bad clue. And Nancy knows she's a great detective. Now she just needs to crack the case. . .