Dead in the Dark - A Maddie Swallows Mystery, #6

Marriage is scary. Dead bodies are worse. Newly engaged Maddie Swallows has done the marriage thing before, and she's terrified to do it again. Even with someone as perfect as Benji. In an attempt to help calm her nerves, Maddie's mother arranges for Maddie, Benji, and the kids to go on a road trip to Carlsbad Caverns. But what begins as a wonderful adventure quickly takes a horrifying turn when, at the bottom of the caverns, the lights go out.
When they turn back on, their tour guide is dead. Trapped with a murderer in the deepest part of the caverns, with no ability to contact help, Maddie and her family must pull together to find the killer so their group gets out safely, and there isn't a second victim.