The Library of Borrowed Hearts by Lucy Gilmore

Librarian Chloe Sampson has been struggling: to take care of her three younger siblings, to find herself, to make ends meet. She's just about at the end of her rope when she stumbles across a rare edition of a book from the 1960s. Deciding it's a sign of her luck turning, she takes it home with her―only to be shocked when her cranky hermit of a neighbor swoops in and offers to buy it for an exorbitant price. Intrigued, Chloe takes a closer look at the book only to find notes scribbled in the margins between two young lovers back when the book was new…one of whom is almost definitely Jasper Holmes, the curmudgeon next door.

When she begins following the clues left behind, she discovers this isn't the only old book in town filled with romantic marginalia. This kickstarts a literary scavenger hunt that Chloe is determined to see through to the end. What happened to the two tragic lovers who corresponded in the margins of so many different library books? And what does it have to do with the old, sad man next door―who only now has begun to open his home and heart to Chloe and her siblings?