Partners in Crime Series by T.K. Eldridge

Detectives Kennedy and Donovan were partners for years. Then one of them died. And yet, they're still partners. Kennedy has to solve Donovan's death while his partner's ghost helps him. Sort of. How do you explain where you got the tips? How do you avoid showing them that you're speaking to thin air and no, you haven't lost your mind? Kennedy has to hide how he's figuring it all out so he can keep his badge and solve the murder - before he ends up dead and buried too.