The Boy Next Door by Josh Lanyon

For the last fourteen years, former celebrated Boy Detective Merle Madison has been trying to build a grown-up career for himself as a private investigator. There are just two problems: there’s not a lot of serious crime in the small town of Hayvenhurst—and Police Chief Isaac Ramsay keeps denying Merle’s application for a PI license.
Merle and Isaac have history, some good and some bad, so when someone seems determined to put Merle out of business—permanently—he naturally turns to his former sidekick for help.

But Isaac’s days of playing second fiddle to a Junior Sherlock Holmes are long past. In fact, Merle will be lucky if Isaac doesn’t kill him himself.