The spare room / Andrea Bartz.

When the pandemic first hits, thirty-one-year-old Kelly tries to make the best of quarantine-but it soon becomes clear there are some lemons you just can't make into lemonade. She loses her job and her boyfriend in one fell swoop, and she finds herself looking for any way to get out of the apartment she's still sharing with her now-ex. So when she rekindles a friendship with a bestselling author she knows from childhood, who invites Kelly to stay with her and her husband in their Virginia mansion, she leaps at the chance. And much to Kelly's surprise, she soon realizes she's developed a bit of a crush-on both of her hosts. Even more unexpected is that both of them seem to reciprocate her feelings-and they say it's the first time they've ever brought someone else into their relationship. But that doesn't quite add up with all of the evidence Kelly keeps finding around the house. Another woman clearly has been there before her, and Kelly really can't understand what reason they would have to lie about it...that is, until she discovers the woman is missing. Cut off from the world, intensely in love, and in way over her head, Kelly is trying desperately to uncover the truth-but the truth might just be that three is a dangerous number.