Wish I Were Here by Melissa Wiesner

In this witty and charming love story perfect for fans of Sophie Cousens, Ashley Poston, and Josie Silver, a buttoned-up math professor is forced to rely on her carefree doorman when her identity mysteriously disappears.

Catherine Lipton carefully calculates everything, and not just because she’s a math professor. She had a chaotic childhood growing up with a free-spirited single dad. So now, from her daily to-dos to her afternoon snacks, Catherine has a plan for it all. But sometimes she wishes she could be someone else, someone with a totally different life. Until suddenly her entire identity—from her Social Security number to her driver’s license to her academic record—mysteriously disappears. There’s no evidence Catherine Lipton ever existed.
With no ID and no other options, Catherine reluctantly accepts help from her exasperatingly laid-back—and infuriatingly attractive—doorman, Luca Morelli. Before long, by-the-books Catherine finds herself bending all the rules with the charismatic Luca—from taking meetings in smoky bars to breaking into hospital record rooms—and having a surprising amount of fun. As Catherine unravels the truth behind her identity’s disappearance, she may discover that the real Catherine has been missing for a lot longer than she realized.