The Burglar in the Closet — Lawrence Block

At a routine teeth-cleaning Bernie Rhodenbarr discovers a few things. One, he's got a nasty cavity. Two, his dentist is unhappily married. Three, his dentist knows Bernie's a burglar. Seems good old Dr. Sheldrake is in need of a burglar to steal back some valuable diamonds from the soon-to-be-ex-Mrs. Sheldrake.
Next thing Bernie realizes he's prowling around Crystal Sheldrake's apartment when someone comes through the door. With only one place to hide, Bernie slips into the closet. Time passes, he emerges, and there's the lovely Crystal Sheldrake lying dead on the floor. To top it all off, the diamonds are gone. With his nemesis Detective Ray Kirschmann hot on his trail, Bernie starts chatting up suspects, chasing down a killer...and wishing he had just stayed in the closet.