Death at Morning House by Maureen Johnson

The fire wasn’t Marlowe Wexler’s fault. Dates should be hot, but not hot enough to warrant literal firefighters. Akilah, the girl Marlowe has been in love with for years, will never go out with her again. No one dates an accidental arsonist.
 
With her house-sitting career up in flames, it seems the universe owes Marlowe a new summer job, and that’s how she ends up at Morning House, a mansion built on an island in the 1920s and abandoned shortly thereafter. It’s easy enough, giving tours. Low risk of fire. High chance of getting bored talking about stained glass and nut cutlets and Prohibition.
 
Oh, and the deaths. Did anyone mention the deaths?
 
Maybe this job isn’t such a gift after all. Morning House has a horrific secret that’s been buried for decades, and now the person who brought her here is missing.
All it takes is one clue to set off a catastrophic chain of events. One small detail, just like a spark, could burn it all down—if someone doesn’t bury Marlowe first.
 
From the bestselling author of the Truly Devious books, Maureen Johnson, comes a new stand-alone YA about a teen who uncovers a mystery while working as a tour guide on an island and must solve it before history repeats itself.
 
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"A stand-alone sapphic mystery following two equally gripping time lines: the Ralston family deaths and Marlowe’s present-day sleuthing. Marlowe’s excellent memory and love of puzzles serve her well, while her self-deprecating, deadpan humor fills the narrative with quirky charm. Suspenseful, smartly executed, and hilarious." — Booklist (starred review)