Dead Letter - M Falcon Mystery, #1

Martin Davies and Della Rose are more interested in having fun playing detective in their golden years than being detectives. Their team of enthusiastic blue-hairs is a treasure trove of unexpected skills and employ unconventional tactics. Behind the sweet wheelchair-bound grandmother facade, Estelle is a master hacker with the online handle MeatGrinder56. Wendy, a semi-retired actress, lives her dream of being a superspy while scrapbooking fingerprints to go with her surveillance photos.
Homeless veteran Mac, the team's eyes on the street, is invisible to the public. When a letter arrives from a deceased client asking them to retrieve a cheap china doll, the game turns fatal as they find more dead bodies than answers. When the killer decides that Della and Martin are getting too close, the couple needs to find the murderer before their fun comes to a permanent end.