To clarify – mundane as in "not supernatural", not boring.
My wife is really into mystery / crime novels. She likes the ones with sort of an eerie, supernatural feel to them, but she finds it kind of boring when the actual final solution is something supernatural – she prefers it to be something realistic that was elaborately covered up to look supernatural – like a more complex Scooby Doo scenario.
By way of examples, only a few come to mind – she really liked Cemetary Dance, liked Brimstone quite a lot, just read Bloodless but didn't enjoy it as much really cause it didn't entirely meet the criteria. Hound of the Baskervilles is another comparison. Some episodes of the TV show Psych fulfilled it (though obviously those were on the goofier side).
In addition to the the main criteria above – she's not super big on romance in mystery stories, especially dislikes infidelity of any sort. Really she's not a huge fan of ones that go into great detail about a detective's personal life in general, preferring it to focus on the mystery – this is also because she likes to be able to hop around in these sorts of books so even if it's a series, not needing to have read every entry is preferable because she'd rather grab the ones with the most appealing premises.
I can try to provide additional info if necessary but I feel I've narrowed it a lot already. If you have something that meets the main criteria but not the others, feel free to recommend it regardless, though if you can mention that I'd really appreciate it! Thanks so much in advance for any help.
Tldr: mysteries that seem like they have a supernatural solution, but it's just an elaborate trick. Prefer: no romance, no infidelity, no real elaborate segments about the detectives personal life (focused on the mystery itself), if a series prefer readable in any order. Cemetary Dance, Brimstone, Hound of the Baskervilles are her best examples of what she's looking for.
by mercury-shade