Janie’s Private Eyes by Zilpha Keatley Snyder, 1989.

This is part of the Stanley Family mysteries series.

Janie Stanley has decided to open her own detective agency, The J.V. Stanley Agency, Incorporated, Private Eyes, with the help of her younger siblings and some other friends.  Eight-year-old Janie has had many different aspirations in her young life, from being a Shakespearean actress to being a vampire, so her older brother, David, doesn’t take her detective games too seriously at first.

However, at Mr. and Mrs. Stanley’s New Year’s Eve party, Janie’s “investigations” come to everyone’s attention when she borrows David’s tape recorder to make audio recordings of guests talking and plays them on the stereo that has been hooked up to speakers to play music for the party.  Gossipy Mrs. Dorfman recognizes her own voice, saying uncomplimentary things about her hosts and the other guests, and leaves in a huff of embarrassment.  When Janie’s father confronts her over the incident, Janie says that she was trying to find evidence on a murderer.  When her father and David question her further, she says that old Mr. Rupert, the deceased father of the Mr. Rupert who owns the local grocery store in Steven’s Corners, was murdered.

When old Mr. Rupert died around Thanksgiving, all the kids in the area were sad.  He was always nice to kids when they were in the grocery store and would sometimes give them candy for free.  They called him Grandpa Rupert.  But, Grandpa Rupert’s death was a heart attack, natural causes.  When David asks Janie what makes her think it was murder, she says that she suspects his son and his wife because they inherited the grocery store.  David says that he doesn’t think that Al Rupert would have killed his father.  Janie also says that she heard that there was no autopsy after the death, and she thinks that’s suspicious.  David says that it was well-known that Grandpa Rupert had heart trouble, so a heart attack wasn’t unexpected.  Janie also says that Huy, the younger brother of her friend, Thuy Tran, saw the mailman talking to Grandpa Rupert just before he died, but David doesn’t see why that’s so suspicious.  Eventually, David and her father talk Janie out of her murder investigation idea, but it turns out that the “murder” wasn’t the only investigation that Janie has undertaken.

Dogs in the area have been disappearing, and the Tran family has come under suspicion.  The Trans haven’t been living in the United States for very long.  Originally, they came from Vietnam.  The reason why people are looking at them suspiciously is because dogs started disappearing around the time the Trans moved to the area, and there are rumors that Vietnamese people eat dogs. (Hint: No, the Trans aren’t eating dogs. That’s an old stereotype/rumor that’s been used against various immigrant groups, and no dogs are eaten in this story.)  Janie knows that the Tran family isn’t guilty of dognapping, but proving it is another matter.  After the trouble at the New Year’s Eve party, she asks David and Amanda to help her investigate.  At first, they don’t want to, but David does have to do a journalism project for school with a partner, Pete Garvey.  Pete Garvey is the school bully who also has a crush on Amanda (which is established in a previous book in the series), but he likes the idea of interviewing people about their missing dogs.

However, even though David, Amanda, and Pete Garvey begin talking to people about the missing dogs, Janie and the other members of her detective agency are still on the case!  David has great misgiving about Janie’s involvement.  Then, suddenly, Pete doesn’t want to work on the project anymore and starts behaving suspiciously.  What does he know that no one else does?

The book is available online through Internet Archive.