I read this as a Kindle book. Unfortunately, I read dozens of Kindle books in a given year, and I have even more in my library that I have not read but picked them up for free. I think this was one of the ones I got for free, somewhere in 2015 or 2016. It was a book of decent length, not a novelette or short story.
The main character was female, an adult, who largely operates out of her van. She has a female college student as an unpaid intern and I think an older male boss. At the start of the book, she's working with a family (mother, father, young kid, I think a girl) who've moved into an old house and are trying to remodel it into a bed-and-breakfast, but have been having issues with the workers getting scared off, the locked door into the unused part of the house unlocking in the middle of the night, and the child being visited by a dark woman at night. The father is skeptical, as typical in hauntings, but the protagonist also notes some other details that make her think that the family had put their remaining assets into the remodeling and he's worried about them losing everything to a scam. I believe we, the readers, then find out that the ghost-hunting business is far from profitable, as it requires a lot of specialized equipment and the organization often gets stiffed when homeowners refuse to render payment, which leads to her realizing this might become another charity case.
She does investigate the house. Her methodology is fairly scientific, involving thermal cameras, motion sensors, and devices to find electromagnetic disruptions. The disturbances are primarily in the locked side of the house and she quickly ties it to a particular case, a wife who hung herself in the main hall, apparently in a bout of religious fervor, after poisoning her husband, although a number of other deaths of transients during the time the house was abandoned have left other ghosts, including one which was a drug addict who suffered an overdose and a prostitute who was killed by her john. I remember there was also some plot point involving her noting that some of the doors had been nailed shut from the outside.
After proving the existence of the ghost to the family, she stages a mock funeral to draw out the ghost (apparently a standard tactic) and manages to trap it in a jar built for holding ghosts (these jar traps typically have a candle in them, which the ghosts will go after to pull out its energy, and can be either manually triggered or set to go off when the heat drops in them). She then takes the ghost to an abandoned graveyard where she buries the jar, forcing the ghost to remain there. If I recall correctly, the ghosts eventually fade away if left there. Unfortunately, the incidents continue at the house, and even get worse. She winds up taking a trip to an abandoned mental asylum in the dead of night to research an aspect of the captured ghost's history, which leads to her encountering ghosts of former mental patients, many of which have gone more crazy than they were before due to the isolation (which reminds me, ghosts can sap electricity for energy, which results in her and her intern losing their flashlights at one point), but they find some of the wife's mortal effects, which leads them to realize that she may not have been the problem, but rather that she was keeping something else quiet. Furthermore, the poisoned husband was apparently involved in some very criminal activities and might have been a devil worshipper.
She goes back to the graveyard and allows the ghost to possess her. Coming back to the house, she battles her way through an increasing number of ghosts, wielding what is essentially an overpowered backpack-powered flashlight to keep the ghosts at bay. Eventually, she breaks into a hidden room, which leads down a crypt filled with the decayed corpses of ritually murdered women, and finds that the aforementioned murdered husband is still alive in a manner of speaking as a result of dark rituals, and plans to use her body to escape. The backpack flashlight gets broken, and I think the main bad guy succumbs to his statue of dark power being broken, and the evil is vanquished.
At the end of the book, she reveals that she learned something else in the process, that the ghost wife had not actually committed suicide, but had been murdered by two handymen, who now own a bar on the edge of town (the protagonist had gotten some information from them before). She reveals that she knows what they did, and then unleashes the ghost to torment them before leaving.
I'm pretty sure it was the beginning of a series. Other details I remember, but can't place the exact location:
- She'd had her own paranormal experience courtesy of a pyrokinetic ghost who burned down her family home when she was a child. I think there's an implication that he's still around before she reveals at the end that she sealed him away a long time ago.
- At some point, she visits a woman in a nursing home who knew the deceased husband (who, come to think of it, was nicknamed "Captain" or something like that). She came away from it feeling that there was something wrong with the narrative, but also with information that the Captain had been less clean-cut than he seemed, and that his presence had been felt in the house after his death. I think it's eventually revealed that the woman was intentionally hiding worse details that she knew about.
- She has a lawyer friend who looks up some information for her in the legal files and provides more as a result of societal gossip (he's not only rich, but also Old Family).
- After the asylum incident, the intern almost quits, having thought of it all as exciting stuff, not something where she could actually get killed, or worse.