Never Trust a Dead Man (by Vivian Vande Velde)

But Selwyn messes up the spell, and Farold's spirit ends up in the body of a bat. Worse, he doesn't know who killed him. So Selwyn, disguised as a Pilgrim, and Farold, in the body of a bat, head back to the village to try to solve the mystery, bickering with and sniping at each other all the way.

Selwyn may be the protagonist, but it's Farold (and Elswyth, when she's around) who holds the reader's attention. Selwyn is just the straight-man to Farold's delightful combination of petulance, snide sarcasm, whiny self-centeredness, deviousness, and good sense, all coming out of the mouth of a bat (and later a goldfinch, and then a duck).

Vivian Vande Velde has taken a classic mystery (red herrings, multiple suspects, a man wrongly accused) and added a touch of magic, a bit of the supernatural, and a dark sense of humor. The result is the author's specialty -- lightweight fun from start to finish. The mystery is nicely balanced to provide clues without being too predictable, and when the mystery slows, the humor fills in to keep things humming along.