It’s not just physical attraction that brings on that first brush of breathlessness in romance. While what we see certainly helps things along, it’s the promise of what remains unseen that keeps passion simmering. This month, three excellent romance novels reveal the myriad ways that mystery begets passion.

"That Mistletoe Moment," by Cat Johnson, Kate Angell and Allyson Charles (Kensington)

Secrecy edges into white lies in the utterly charming That Mistletoe Moment (Kensington), a collection of Christmas-themed novellas from Cat Johnson, Kate Angell and Allyson Charles. The stories hang together with an amusing conceit: the Build-a-Boyfriend app. Users set up an account, design their perfect fake boyfriend, and receive texts, pictures, emails, even flowers all through the holiday season; it’s the perfect antidote to nosy relatives. Of course, in romance, an app such as this one is destined to make things more complicated, and it does: Johnson’s “A Boyfriend by Christmas,” is a case of mistaken identity when heroine Noelle designs a fake boyfriend based on (and named for) a real-life man and accidentally begins a real relationship. In Angell’s “All I Want for Christmas Is . . .” the heroine vies for a position working for a crusty CEO and joins the app when he insists he will hire only assistants who are in committed relationships to avoid even the hint of impropriety — landing them in a delightful mess when he decides he’d like very much to be improper. Charles’s “Her Favorite Present” wraps up the collection when the cynical founder of Build-a-Boyfriend finds himself falling for one of his customers, and discovering that love isn’t as much of an illusion as he once believed.

"Deadly Silence," by Rebecca Zanetti (Forever)

Revelations of the past are at the heart of Deadly Silence (Forever), the first installment of Rebecca Zanetti’s Blood Brothers series — a spinoff of her immensely popular Sin Brothers series. Raised in a horrific home for boys, Ryker Jones and his two brothers are genetic anomalies, born with superior skills and strengths that make them excellent private investigators, specializing in missing persons. They land in Wyoming on the hunt for a serial killer — and on the run from those who ran the home where they were raised. Though the brothers have made it a rule to avoid relationships, Ryker can’t seem to ignore his growing feelings for local paralegal Zara Remington, entwined in a mystery of her own. Zanetti balances the adventure and menace of Zara and Ryker’s lives with a relatable romance. The result is a story that’s sexy and emotional, and filled with a rich look at love in all its forms.

"The Soldier's Scoundrel," by Cat Sebastian (Avon Impulse/Avon Impulse)

In the historical romance The Soldier’s Scoundrel (Avon Impulse), Cat Sebastian sets a beautiful romance against a compelling story of blackmail, scandal and the impossibility of happily-ever-after. A former thief who was born into poverty, Jack Turner is now an investigator who devotes himself to helping those who need it most in 19th-century London. When Oliver Rivington, second son to an earl, demands to know why his sister has had business dealings with Jack, who works exclusively outside the law, the detective wants nothing to do with the snobbish, moneyed aristocrat. If only it were so easy. Jack and Oliver are drawn to each other almost immediately. To be together, the men must tackle their own prejudice and that of society — sodomy laws made public single-sex relationships illegal during the time period. Nevertheless, Jack and Oliver find themselves willing to do anything to be together, a reminder that love holds such power over us all.

Sarah MacLean reviews romance novels monthly for The Washington Post. Her most recent book is “A Scot in the Dark.”