Love Theoretically by Ali Hazelwood:

The many lives of theoretical physicist Elsie Hannaway have finally caught up with her. By day, she’s an adjunct professor, toiling away at grading labs and teaching thermodynamics in the hopes of landing tenure. By other day, Elsie makes up for her non-existent paycheck by offering her services as a fake girlfriend, tapping into her expertly honed people pleasing skills to embody whichever version of herself the client needs.

Honestly, it’s a pretty sweet gig—until her carefully constructed Elsie-verse comes crashing down. Because Jack Smith, the annoyingly attractive and broody older brother of her favorite client, turns out to be the cold-hearted experimental physicist who ruined her mentor’s career and undermined the reputation of theorists everywhere. And that same Jack who now sits on the hiring committee at MIT, right between Elsie and her dream job.

Elsie is prepared for an all-out war of scholarly sabotage but…those long, penetrating looks? Not having to be anything other than her true self when she’s with him? Will falling into an experimentalist’s orbit finally tempt her to put her most guarded theories on love into practice?